TY - GEN A1 - Barniske, Andreas A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - Two extremely luminous WN stars in the Galactic center with circumstellar emission from dust and gas (vol 486, pg 971, 2008) T2 - Physical chemistry, chemical physics : a journal of European Chemical Societies KW - stars: Wolf-Rayet KW - HII regions KW - Galaxy: center KW - stars: individual: WR 102ka KW - stars: individual: WR 102c KW - errata, addenda Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/200809568e SN - 1432-0746 VL - 587 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Bozzo, Enrico A1 - Oskinova, Lida A1 - Lobel, A. A1 - Hamann, Wolf-Rainer T1 - The super-orbital modulation of supergiant high-mass X-ray binaries JF - Astronomy and astrophysics : an international weekly journal N2 - The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries. KW - X-rays: stars KW - X-rays: binaries KW - gamma rays: stars KW - stars: massive KW - stars: neutron Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201731930 SN - 1432-0746 VL - 606 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Brown, John C. A1 - Barrett, R. K. A1 - Oskinova, Lida A1 - Owocki, S. P. A1 - Hamann, Wolf-Rainer A1 - de Jong, J. A. A1 - Kaper, L. A1 - Henrichs, H. F. T1 - Inference of hot star density stream properties from data on rotationally recurrent DACs N2 - The information content of data on rotationally periodic recurrent discrete absorption components (DACs) in hot star wind emission lines is discussed. The data comprise optical depths tau(w,phi) as a function of dimensionless Doppler velocity w=(Deltalambda/lambda(0))(c/v(infinity)) and of time expressed in terms of stellar rotation angle phi. This is used to study the spatial distributions of density, radial and rotational velocities, and ionisation structures of the corotating wind streams to which recurrent DACs are conventionally attributed. The simplifying assumptions made to reduce the degrees of freedom in such structure distribution functions to match those in the DAC data are discussed and the problem then posed in terms of a bivariate relationship between tau(w, phi) and the radial velocity v(r)(r), transverse rotation rate Omega(r) and density rho(r, phi) structures of the streams. The discussion applies to cases where: the streams are equatorial; the system is seen edge on; the ionisation structure is approximated as uniform; the radial and transverse velocities are taken to be functions only of radial distance but the stream density is allowed to vary with azimuth. The last kinematic assumption essentially ignores the dynamical feedback of density on velocity and the relationship of this to fully dynamical models is discussed. The case of narrow streams is first considered, noting the result of Hamann et al. (2001) that the apparent acceleration of a narrow stream DAC is higher than the acceleration of the matter itself, so that the apparent slow acceleration of DACs cannot be attributed to the slowness of stellar rotation. Thus DACs either involve matter which accelerates slower than the general wind flow, or they are formed by structures which are not advected with the matter flow but propagate upstream (such as Abbott waves). It is then shown how, in the kinematic model approximation, the radial speed of the absorbing matter can be found by inversion of the apparent acceleration of the narrow DAC, for a given rotation law. The case of broad streams is more complex but also more informative. The observed tau(w,phi) is governed not only by v(r)(r) and Omega(r) of the absorbing stream matter but also by the density profile across the stream, determined by the azimuthal (phi(0)) distribution function F- 0(phi(0)) of mass loss rate around the stellar equator. When F-0(phi(0)) is fairly wide in phi(0), the acceleration of the DAC peak tau(w, phi) in w is generally slow compared with that of a narrow stream DAC and the information on v(r)(r), Omega(r) and F-0(phi(0)) is convoluted in the data tau(w, phi). We show that it is possible, in this kinematic model, to recover by inversion, complete information on all three distribution functions v(r)(r), Omega(r) and F- 0(phi(0)) from data on tau(w, phi) of sufficiently high precision and resolution since v(r)(r) and Omega(r) occur in combination rather than independently in the equations. This is demonstrated for simulated data, including noise effects, and is discussed in relation to real data and to fully hydrodynamic models Y1 - 2004 SN - 0004-6361 ER - TY - JOUR A1 - Burgemeister, S. A1 - Gvaramadze, Visily V. A1 - Stringfellow, G. S. A1 - Kniazev, Alexei Y. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer T1 - WR 120bb and WR 120bc: a pair of WN9h stars with possibly interacting circumstellar shells JF - Monthly notices of the Royal Astronomical Society N2 - Two optically obscured Wolf-Rayet (WR) stars have been recently discovered by means of their infrared (IR) circumstellar shells, which show signatures of interaction with each other. Following the systematics of the WR star catalogues, these stars obtain the names WR 120bb and WR 120bc. In this paper, we present and analyse new near-IR, J-, H- and K-band spectra using the Potsdam Wolf-Rayet model atmosphere code. For that purpose, the atomic data base of the code has been extended in order to include all significant lines in the near-IR bands. The spectra of both stars are classified as WN9h. As their spectra are very similar the parameters that we obtained by the spectral analyses hardly differ. Despite their late spectral subtype, we found relatively high stellar temperatures of 63 kK. The wind composition is dominated by helium, while hydrogen is depleted to 25 per cent by mass. Because of their location in the Scutum-Centaurus Arm, WR 120bb and WR 120bc appear highly reddened, A(Ks) approximate to 2 mag. We adopt a common distance of 5.8 kpc to both stars, which complies with the typical absolute K-band magnitude for the WN9h subtype of -6.5 mag, is consistent with their observed extinction based on comparison with other massive stars in the region, and allows for the possibility that their shells are interacting with each other. This leads to luminosities of log(L/L-circle dot) = 5.66 and 5.54 for WR 120bb and WR 120bc, with large uncertainties due to the adopted distance. The values of the luminosities of WR 120bb and WR 120bc imply that the immediate precursors of both stars were red supergiants (RSG). This implies in turn that the circumstellar shells associated with WR 120bb and WR 120bc were formed by interaction between the WR wind and the dense material shed during the preceding RSG phase. KW - line: identification KW - circumstellar matter KW - stars: fundamental parameters KW - stars: massive KW - stars: Wolf-Rayet Y1 - 2013 U6 - https://doi.org/10.1093/mnras/sts588 SN - 0035-8711 SN - 1365-2966 VL - 429 IS - 4 SP - 3305 EP - 3315 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Corcoran, Michael F. A1 - Nichols, Joy S. A1 - Pablo, Herbert A1 - Shenar, Tomer A1 - Pollock, Andy M. T. A1 - Waldron, Wayne L. A1 - Moffat, Anthony F. J. A1 - Richardson, Noel D. A1 - Russell, Christopher M. P. A1 - Hamaguchi, Kenji A1 - Huenemoerder, David P. A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Naze, Yael A1 - Ignace, Richard A1 - Evans, Nancy Remage A1 - Lomax, Jamie R. A1 - Hoffman, Jennifer L. A1 - Gayley, Kenneth A1 - Owocki, Stanley P. A1 - Leutenegger, Maurice A1 - Gull, Theodore R. A1 - Hole, Karen Tabetha A1 - Lauer, Jennifer A1 - Iping, Rosina C. T1 - A coordinated X-Ray and optical campaign of the nearest massive eclipsing binary, delta ORIONIS Aa. I. Overview of thr X-Ray spectrum JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present an overview of four deep phase-constrained Chandra HETGS X-ray observations of delta Ori A. Delta Ori A is actually a triple system that includes the nearest massive eclipsing spectroscopic binary, delta Ori Aa, the only such object that can be observed with little phase-smearing with the Chandra gratings. Since the fainter star, delta Ori Aa2, has a much lower X-ray luminosity than the brighter primary (delta Ori Aa1), delta Ori Aa provides a unique system with which to test the spatial distribution of the X-ray emitting gas around delta Ori Aa1 via occultation by the photosphere of, and wind cavity around, the X-ray dark secondary. Here we discuss the X-ray spectrum and X-ray line profiles for the combined observation, having an exposure time of nearly 500 ks and covering nearly the entire binary orbit. The companion papers discuss the X-ray variability seen in the Chandra spectra, present new space-based photometry and ground-based radial velocities obtained simultaneously with the X-ray data to better constrain the system parameters, and model the effects of X-rays on the optical and UV spectra. We find that the X-ray emission is dominated by embedded wind shock emission from star Aa1, with little contribution from the tertiary star Ab or the shocked gas produced by the collision of the wind of Aa1 against the surface of Aa2. We find a similar temperature distribution to previous X-ray spectrum analyses. We also show that the line half-widths are about 0.3-0.5 times the terminal velocity of the wind of star Aa1. We find a strong anti-correlation between line widths and the line excitation energy, which suggests that longer-wavelength, lower-temperature lines form farther out in the wind. Our analysis also indicates that the ratio of the intensities of the strong and weak lines of Fe XVII and Ne X are inconsistent with model predictions, which may be an effect of resonance scattering. KW - binaries: close KW - binaries: eclipsing KW - stars: early-type KW - stars: individual (Delta Ori) KW - stars: mass-loss KW - X-rays: stars Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/809/2/132 SN - 0004-637X SN - 1538-4357 VL - 809 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - DeMarco, O. A1 - Schmutz, W. A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - Gamma 2 Velorum revisited Y1 - 1999 ER - TY - JOUR A1 - DeMarco, O. A1 - Schmutz, W. A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer A1 - DeMarco, O. A1 - DeKoter, A. T1 - Why should we compare WR codes? Y1 - 1999 ER - TY - GEN A1 - Dolezalova, Barbora A1 - Kubatova, Brankica A1 - Kubat, Jiri A1 - Hamann, Wolf-Rainer T1 - The Quasi-WR Star HD 45166 Revisited T2 - Radiative signatures from the cosmos N2 - We studied the wind of the quasi Wolf-Rayet (qWR) star HD 45166. As a first step we modeled the observed UV spectra of this star by means of the state-of-the-art Potsdam Wolf-Rayet (PoWR) atmosphere code. We inferred the wind parameters and compared them with previous findings. Y1 - 2019 SN - 978-1-58381-925-8 SN - 1050-3390 VL - 519 SP - 197 EP - 200 PB - Astronomical soc pacific CY - San Fransisco ER - TY - JOUR A1 - Evans, C. J. A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Gallagher, J. S. A1 - Chu, Y.-H. A1 - Gruendl, R. A. A1 - Hamann, Wolf-Rainer A1 - Henault-Brunet, V. A1 - Todt, Helge Tobias T1 - A rare early-type star revealed in the wing of the small megellanic cloud JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Sk 183 is the visually brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption, which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere four. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46 +/- 2 kK, a low mass-loss rate of similar to 10(-7) M-circle dot yr(-1), and a spectroscopic mass of 46(-8)(+ 9) M-circle dot (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (similar to 47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionizing photons. This suggests Sk 183 is the primary influence on the star formation along the inner edge of the nebula. KW - open clusters and associations: individual (NGC 602) KW - stars: early-type KW - stars: fundamental parameters KW - stars: individual (Sanduleak 183) Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/753/2/173 SN - 0004-637X VL - 753 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Evans, C. J. A1 - Smartt, S. J. A1 - Lee, J. K. A1 - Lennon, D. J. A1 - Kaufer, A. A1 - Dufton, P. L. A1 - Trundle, C. A1 - Herrero, A. A1 - Simon Díaz, Sergio A1 - de Koter, A. A1 - Hamann, Wolf-Rainer A1 - Hendry, M. A. A1 - Hunter, I. A1 - Irwin, M. J. A1 - Korn, A. J. A1 - Kudritzki, R. P. A1 - Langer, Norbert A1 - Mokiem, M. R. A1 - Najarro, F. A1 - Pauldrach, A. W. A. A1 - Przybilla, Norbert A1 - Puls, J. A1 - Ryans, R. S. I. A1 - Urbaneja, M. A. A1 - Venn, K. A. A1 - Villamariz, M. R. T1 - The VLT-FLAMES survey of massive stars : Observations in the Galactic clusters NGC3293, NGC4755 and NGC6611 N2 - We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi- Element Spectrograph ( FLAMES) instrument at the Very Large Telescope ( VLT). Here we present observations of 269 Galactic stars with the FLAMES- Giraffe Spectrograph ( R similar or equal to 25 000), in fields centered on the open clusters NGC3293, NGC4755 and NGC6611. These data are supplemented by a further 50 targets observed with the Fibre- Fed Extended Range Optical Spectrograph ( FEROS, R = 48 000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC3293 and NGC4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared Y1 - 2005 ER - TY - JOUR A1 - Fang, X. A1 - Guerrero, Martín A. A1 - Marquez-Lugo, R. A. A1 - Toala, Jesús Alberto A1 - Arthur, S. J. A1 - Chu, Y.-H. A1 - Blair, William P. A1 - Gruendl, R. A. A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Todt, Helge Tobias T1 - Expansion of hydrogen-poor knots in the born-again planetary nebulae A30 and A78 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We analyze the expansion of hydrogen-poor knots and filaments in the born-again planetary nebulae A30 and A78 based on Hubble Space Telescope ( HST) images obtained almost 20 yr apart. The proper motion of these features generally increases with distance to the central star, but the fractional expansion decreases, i.e., the expansion is not homologous. As a result, there is not a unique expansion age, which is estimated to be 610-950 yr for A30 and 600-1140 yr for A78. The knots and filaments have experienced complex dynamical processes: the current fast stellar wind is mass loaded by the material ablated from the inner knots; the ablated material is then swept up until it shocks the inner edges of the outer, hydrogen-rich nebula. The angular expansion of the outer filaments shows a clear dependence on position angle, indicating that the interaction of the stellar wind with the innermost knots channels the wind along preferred directions. The apparent angular expansion of the innermost knots seems to be dominated by the rocket effect of evaporating gas and by the propagation of the ionization front inside them. Radiation-hydrodynamical simulations show that a single ejection of material followed by a rapid onset of the stellar wind and ionizing flux can reproduce the variety of clumps and filaments at different distances from the central star found in A30 and A78. KW - ISM: kinematics and dynamics KW - planetary nebulae: individual (A30 and A78) Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/797/2/100 SN - 0004-637X SN - 1538-4357 VL - 797 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - CHAP A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer A1 - Rätzel, D. A1 - Oskinova, Lida T1 - Hydrodynamic simulations of clumps N2 - Clumps in hot star winds can originate from shock compression due to the line driven instability. One-dimensional hydrodynamic simulations reveal a radial wind structure consisting of highly compressed shells separated by voids, and colliding with fast clouds. Two-dimensional simulations are still largely missing, despite first attempts. Clumpiness dramatically affects the radiative transfer and thus all wind diagnostics in the UV, optical, and in X-rays. The microturbulence approximation applied hitherto is currently superseded by a more sophisticated radiative transfer in stochastic media. Besides clumps, i.e. jumps in the density stratification, so-called kinks in the velocity law, i.e. jumps in dv/dr, play an eminent role in hot star winds. Kinks are a new type of radiative-acoustic shock, and propagate at super-Abbottic speed. Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-17975 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - X-ray line emission from a fragmented stellar wind N2 - We discuss X-ray line formation in dense O star winds. A random distribution of wind shocks is assumed to emit X-rays that are partially absorbed by cooler wind gas. The cool gas resides in highly compressed fragments oriented perpendicular to the radial flow direction. For fully opaque fragments, we find that the blueshifted part of X-ray line profiles remains flat-topped even after severe wind attenuation, whereas the red part shows a steep decline. These box- type, blueshifted profiles resemble recent Chandra observations of the O3 star zeta Pup. For partially transparent fragments, the emission lines become similar to those from a homogeneous wind. Y1 - 2003 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Owocki, S. P. T1 - Overloaded and fractured winds N2 - We discuss the connection between wind overloading and discrete absorption components in P Cygni line profiles from O stars. Overloading can create horizontal plateaus in the radial wind speed that cause the extra absorption in the line profile. The upstream propagation speed of these velocity plateaus is analyzed. The second part of the paper deals with X-ray emission from O stars. X-ray line profiles observed with Chandra and XMM are often symmetric, contrary to what is expected for lines from a homogeneous wind. We discuss the influence on line symmetry of photon escape channels in a strongly clumped wind. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Shlosman, Isak A1 - Hamann, Wolf-Rainer T1 - Runaway acceleration of line-driven winds : the role of the outer boundary N2 - Observations and theory suggest that line driven winds from hot stars and luminous accretion disks adopt a unique, critical solution which corresponds to maximum mass loss rate. We analyze the numerical stability of the infinite family of shallow wind solutions, which resemble solar wind breezes, and their transition to the critical wind. Shallow solutions are sub-critical with respect to radiative (or Abbott) waves. These waves can propagate upstream through shallow winds at high speeds. If the waves are not accounted for in the Courant time step, numerical runaway results. The outer boundary condition is equally important for wind stability. Assuming pure outflow conditions, as is done in the literature, triggers runaway of shallow winds to the critical solution or to accretion flow. Y1 - 2002 ER - TY - JOUR A1 - Fulmer, Leah M. A1 - Gallagher, John S. A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Ramachandran, Varsha T1 - Testing massive star evolution, star-formation history, and feedback at low metallicity BT - photometric analysis of OB stars in the SMC Wing JF - Astronomy and astrophysics : an international weekly journal N2 - Context. The supergiant ionized shell SMC-SGS 1 (DEM 167), which is located in the outer Wing of the Small Magellanic Cloud (SMC), resembles structures that originate from an energetic star-formation event and later stimulate star formation as they expand into the ambient medium. However, stellar populations within and surrounding SMC-SGS 1 tell a different story. Aims. We present a photometric study of the stellar population encompassed by SMC-SGS 1 in order to trace the history of such a large structure and its potential influence on star formation within the low-density, low-metallicity environment of the SMC. Methods. For a stellar population that is physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV) photometry from the Galaxy Evolution Explorer with archival optical (V-band) photometry from the ESO Danish 1.54 m Telescope. Given their colors and luminosities, we estimated stellar ages and masses by matching observed photometry to theoretical stellar isochrone models. Results. We find that the investigated region supports an active, extended star-formation event spanning similar to 25-40 Myr ago, as well as continued star formation into the present. Using a standard initial mass function, we infer a lower bound on the stellar mass from this period of similar to 3 x 10(4) M-circle dot, corresponding to a star-formation intensity of similar to 6 x 10(-3) M-circle dot kpc(-2) yr(-1). Conclusions. The spatial and temporal distributions of young stars encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation over millions of years. Ongoing star formation, both along the edge and interior to SMC-SGS 1, suggests a combined stimulated and stochastic mode of star formation within the SMC Wing. We note that a slow expansion of the shell within this low-density environment may preserve molecular clouds within the volume of the shell, leaving them to form stars even after nearby stellar feedback expels local gas and dust. KW - galaxies KW - stellar content KW - stars KW - formation KW - individual KW - Small KW - Magellanic Cloud Y1 - 2020 U6 - https://doi.org/10.1051/0004-6361/201834314 SN - 0004-6361 SN - 1432-0746 VL - 633 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Geist, Emily A1 - Gallagher, John S. A1 - Kotulla, Ralf A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Ramachandran, Varsha A1 - Sabbi, Elena A1 - Smith, Linda J. A1 - Kniazev, Alexey A1 - Nota, Antonella A1 - Rickard, Matthew J. T1 - Ionization and star formation in the giant H ii region SMC-N66 JF - Publications of the Astronomical Society of the Pacific N2 - The NGC 346 young stellar system and associated N66 giant H ii region in the Small Magellanic Cloud are the nearest example of a massive star-forming event in a low metallicity (Z approximate to 0.2Z (circle dot)) galaxy. With an age of less than or similar to 3 Myr this system provides a unique opportunity to study relationships between massive stars and their associated H ii region. Using archival data, we derive a total H alpha luminosity of L(H alpha) = 4.1 x 10(38) erg s(-1) corresponding to an H-photoionization rate of 3 x 10(50) s(-1). A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from Hubble Space Telescope far-ultraviolet (FUV) spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the FUV luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L(H alpha) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments. Y1 - 2022 U6 - https://doi.org/10.1088/1538-3873/ac697b SN - 0004-6280 SN - 1538-3873 VL - 134 IS - 1036 PB - IOP Publishing CY - Bristol ER - TY - JOUR A1 - Gimenez-Garcia, Ana A1 - Shenar, Tomer A1 - Torrejon, J. M. A1 - Oskinova, Lida A1 - Martinez-Nunez, S. A1 - Hamann, Wolf-Rainer A1 - Rodes-Roca, J. J. A1 - González-Galan, A. A1 - Alonso-Santiago, J. A1 - González-Fernández, C. A1 - Bernabeu, Guillermo A1 - Sander, Andreas Alexander Christoph T1 - Measuring the stellar wind parameters in IGR J17544-2619 and Vela X-1 constrains the accretion physics in supergiant fast X-ray transient and classical supergiant X-ray binaries JF - Siberian Mathematical Journal N2 - Aims. To close this gap, we perform a comparative analysis of the optical companion in two important systems: IGR J175442619 (SFXT) and Vela X-1 (SGXB). We analyze the spectra of each star in detail and derive their stellar and wind properties. As a next step, we compare the wind parameters, giving us an excellent chance of recognizing key differences between donor winds in SFXTs and SGXBs. Methods. We use archival infrared, optical and ultraviolet observations, and analyze them with the non-local thermodynamic equilibrium (NLTE) Potsdam Wolf-Rayet model atmosphere code. We derive the physical properties of the stars and their stellar winds, accounting for the influence of X-rays on the stellar winds. Results. We find that the stellar parameters derived from the analysis generally agree well with the spectral types of the two donors: O9I (IGR J17544-2619) and B0.5Iae (Vela X-1). The distance to the sources have been revised and also agree well with the estimations already available in the literature. In IGR J17544-2619 we are able to narrow the uncertainty to d = 3.0 +/- 0.2 kpc. From the stellar radius of the donor and its X-ray behavior, the eccentricity of IGR J17544-2619 is constrained to e < 0.25. The derived chemical abundances point to certain mixing during the lifetime of the donors. An important difference between the stellar winds of the two stars is their terminal velocities (v(infinity) = 1500 km s(-1) in IGR J17544-2619 and v(infinity) = 700 km s(-1) in Vela X-1), which have important consequences on the X-ray luminosity of these sources. Conclusions. The donors of IGR J17544-2619 and Vela X-1 have similar spectral types as well as similar parameters that physically characterize them and their spectra. In addition, the orbital parameters of the systems are similar too, with a nearly circular orbit and short orbital period. However, they show moderate differences in their stellar wind velocity and the spin period of their neutron star which has a strong impact on the X-ray luminosity of the sources. This specific combination of wind speed and pulsar spin favors an accretion regime with a persistently high luminosity in Vela X-1, while it favors an inhibiting accretion mechanism in IGR J17544-2619. Our study demonstrates that the relative wind velocity is critical in class determination for the HMXBs hosting a supergiant donor, given that it may shift the accretion mechanism from direct accretion to propeller regimes when combined with other parameters. KW - accretion, accretion disks KW - methods: observational KW - techniques: spectroscopic KW - stars: atmospheres KW - X-rays: binaries KW - stars: winds, outflows Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527551 SN - 1432-0746 VL - 591 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gruner, David A1 - Hainich, Rainer A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Todt, Helge Tobias A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Ayres, T. A1 - Hamann, Wolf-Rainer T1 - The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis JF - Astronomy and astrophysics : an international weekly journal N2 - Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system. KW - stars: individual: HD 93129A KW - stars: atmospheres KW - stars: fundamental parameters KW - stars: early-typeP Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833178 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer T1 - Hydrodynamic model atmospheres for WR stars : self-consistent modeling of a WC star wind N2 - We present the first non-LTE atmosphere models for WRstars that incorporate a self-consistent solution of the hydrodynamic equations. The models take iron-group line-blanketing and clumping into account, and compute the hydrodynamic structure of a radiatively driven wind consistently with the non-LTE radiation transport in the co-moving frame. We construct a self-consistent wind model that reproduces all observed properties of an early-type WCstar (WC5). We find that the WR-type mass-loss is initiated at high optical depth by the so-called "Hot Iron Bump" opacities (Fe IX- XVI). The acceleration of the outer wind regions is due to iron-group ions of lower excitation in combination with C and O. Consequently, the wind structure shows two acceleration regions, one close to the hydrostatic wind base in the optically thick part of the atmosphere, and another farther out in the wind. In addition to the radiative acceleration, the "Iron Bump" opacities are responsible for an intense heating of deep atmospheric layers. We find that the observed narrow O VI emission lines in the optical spectra of WC stars originate from this region. From their dependence on the clumping factor we gain important information about the location where the density inhomogeneities in WR-winds start to develop Y1 - 2005 SN - 0004-6361 ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer T1 - Hydrodynamic model atmospheres for hot stars N2 - Recent non-LTE models for expanding atmospheres, accounting for iron group line-blanketing and clumping, show a radiative acceleration which supplies a large part of the driving force of WR winds. Aiming at the calculation of fully consistent wind models, we developed a method to include the solution of the hydrodynamic equations into our code, taking into account the radiation pressure from the comoving-frame radiation transport. In the present work we discuss the resulting wind acceleration for WR- and O star models, and demonstrate the effects of clumping. In addition, we present a consistent hydrodynamic non-LTE model for the O-star zeta Puppis, which is calculated under consideration of complex model atoms of H, He, C, N, O, Si and the iron group elements. In its present state this model fails to reproduce the observed mass loss rate - probably due to still incomplete atomic data. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer T1 - Spectral analysis of the LMC [WC] star SMP 61 N2 - HST UV and optical spectra of the early-type [WC] star SMP 61 in the LMC are analyzed by means of line blanketed non-LTE models for expanding atmospheres. The known distance to the LMC allows a reliable determination of the stellar parameters. The low iron surface abundance of the object possibly indicates a preceding evolution through a very late thermal pulse (VLTP). Y1 - 2003 SN - 1-583-81148-6 ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer A1 - Hillier, D. J. A1 - Koesterke, Lars T1 - Spectral analyses of WC stars in the LMC Y1 - 1998 ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - The impact of iron group elements on the ionizatin structure of WC star atmospheres : WR111 Y1 - 2000 ER - TY - JOUR A1 - Gräfener, Götz A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - Spectral analyses of WC stars in the LMC Y1 - 1999 ER - TY - JOUR A1 - Gräfener, Götz A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - The WR population in CTS 1026 N2 - The blue compact H II galaxy CTS 1026 shows very strong WR emission features around 4686 AA and 5800 AA. We present high S/N optical spectra of the nucleus of this object. Byanalysis of the WR profile shapes, we determine the dominant spectral types and the WN/WC ratio in the starforming region. The ratio WR/O is determined via standard nebular diagnostics. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Gräfener, Götz A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - Line-blanketed model atmospheres for WR star N2 - We describe the treatment of iron group line-blanketing in non-LTE model atmospheres for WR stars. As an example, a blanketed model for the early-type WC star WR 111 is compared to its un-blanketed counterpart. Blanketing affects the ionization structure and the emergent flux distribution of our models. The radiation pressure, as computed within our models, falls short by only a factor of two to provide the mechanical power of the WR wind. Y1 - 2002 ER - TY - JOUR A1 - Guerrero, Martín A. A1 - Ruiz, N. A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Todt, Helge Tobias A1 - Schönberner, Detlef A1 - Oskinova, Lida A1 - Gründl, R. A. A1 - Steffen, M. A1 - Blair, William P. A1 - Toala, Jesús Alberto T1 - Rebirth of X-Ray emission from the born-again planetary Nebula A30 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible. KW - planetary nebulae: general KW - planetary nebulae: individual (A30) KW - stars: winds, outflows KW - X-rays: ISM Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/755/2/129 SN - 0004-637X VL - 755 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Gvaramadze, V. V. A1 - Chene, A.-N. A1 - Kniazev, A. Y. A1 - Schnurr, O. A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Hainich, Rainer A1 - Langer, N. A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Gruendl, R. A. T1 - Discovery of a new Wolf-Rayet star and a candidate star cluster in the Large Magellanic Cloud with Spitzer JF - Monthly notices of the Royal Astronomical Society N2 - We report the first-ever discovery of a Wolf-Rayet (WR) star in the Large Magellanic Cloud via detection of a circular shell with the Spitzer Space Telescope. Follow-up observations with Gemini-South resolved the central star of the shell into two components separated from each other by a parts per thousand 2 arcsec (or a parts per thousand 0.5 pc in projection). One of these components turns out to be a WN3 star with H and He lines both in emission and absorption (we named it BAT99 3a using the numbering system based on extending the Breysacher et al. catalogue). Spectroscopy of the second component showed that it is a B0 V star. Subsequent spectroscopic observations of BAT99 3a with the du Pont 2.5-m telescope and the Southern African Large Telescope revealed that it is a close, eccentric binary system, and that the absorption lines are associated with an O companion star. We analysed the spectrum of the binary system using the non-LTE Potsdam WR (powr) code, confirming that the WR component is a very hot (a parts per thousand 90 kK) WN star. For this star, we derived a luminosity of log L/ L-aS (TM) = 5.45 and a mass-loss rate of 10(- 5.8) M-aS (TM) yr(- 1), and found that the stellar wind composition is dominated by helium with 20 per cent of hydrogen. Spectroscopy of the shell revealed an He iii region centred on BAT99 3a and having the same angular radius (a parts per thousand 15 arcsec) as the shell. We thereby add a new example to a rare class of high-excitation nebulae photoionized by WR stars. Analysis of the nebular spectrum showed that the shell is composed of unprocessed material, implying that the shell was swept-up from the local interstellar medium. We discuss the physical relationship between the newly identified massive stars and their possible membership of a previously unrecognized star cluster. KW - line: identification KW - binaries: spectroscopic KW - stars: massive KW - stars: Wolf-Rayet KW - ISM: bubbles Y1 - 2014 U6 - https://doi.org/10.1093/mnras/stu909 SN - 0035-8711 SN - 1365-2966 VL - 442 IS - 2 SP - 929 EP - 945 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Gvaramadze, V. V. A1 - Kniazev, A. Y. A1 - Miroshnichenko, A. S. A1 - Berdnikov, Leonid N. A1 - Langer, N. A1 - Stringfellow, G. S. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Grebel, E. K. A1 - Buckley, D. A1 - Crause, L. A1 - Crawford, S. A1 - Gulbis, A. A1 - Hettlage, C. A1 - Hooper, E. A1 - Husser, T. -O. A1 - Kotze, P. A1 - Loaring, N. A1 - Nordsieck, K. H. A1 - O'Donoghue, D. A1 - Pickering, T. A1 - Potter, S. A1 - Colmenero, E. Romero A1 - Vaisanen, P. A1 - Williams, T. A1 - Wolf, M. A1 - Reichart, D. E. A1 - Ivarsen, K. M. A1 - Haislip, J. B. A1 - Nysewander, M. C. A1 - LaCluyze, A. P. T1 - Discovery of two new Galactic candidate luminous blue variables with Wide-field Infrared Survey Explorer JF - Monthly notices of the Royal Astronomical Society N2 - We report the discovery of two new Galactic candidate luminous blue variable (LBV) stars via detection of circular shells (typical of confirmed and candidate LBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 mu m in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered candidate LBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R and I bands by 0.68 +/- 0.10 and 0.61 +/- 0.04 mag, respectively, during the last 1318 years, while the second one (known as Hen 3-1383) varies its B, V, R, I and Ks brightnesses by similar or equal to 0.50.9 mag on time-scales from 10 d to decades. We also found significant changes in the spectrum of Hen 3-1383 on a time-scale of similar or equal to 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered candidate LBVs might be directly related to their possible runaway status. KW - line: identification KW - circumstellar matter KW - stars: emission-line, Be KW - stars: evolution KW - stars: individual: Hen 3-1383 KW - stars: massive Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-2966.2012.20556.x SN - 0035-8711 VL - 421 IS - 4 SP - 3325 EP - 3337 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Gvaramadze, Vasily V. A1 - Fabrika, Sergei Nikolaevich A1 - Hamann, Wolf-Rainer A1 - Sholukhova, Olga N. A1 - Valeev, Azamat F. A1 - Goranskij, Vitaly P. A1 - Cherepashchuk, Anatol M. A1 - Bomans, Dominik J. A1 - Oskinova, Lida T1 - Discovery of a new Wolf-Rayet star and its ring nebula in Cygnus N2 - We report the serendipitous discovery of a ring nebula around a candidate Wolf-Rayet (WR) star, HBHA 4202-22, in Cygnus using the Spitzer Space Telescope archival data. Our spectroscopic follow-up observations confirmed the WR nature of this star (we named it WR 138a) and showed that it belongs to the WN8-9h subtype. We thereby add a new example to the known sample of late WN stars with circumstellar nebulae. We analysed the spectrum of WR 138a by using the Potsdam Wolf-Rayet (PoWR) model atmospheres, obtaining a stellar temperature of 40 kK. The stellar wind composition is dominated by helium with 20 per cent of hydrogen. The stellar spectrum is highly reddened and absorbed (EB- V = 2.4 mag, A(V) = 7.4 mag). Adopting a stellar luminosity of log L/L-circle dot = 5.3, the star has a mass-loss rate of 10-4.7 M- circle dot yr-1, and resides in a distance of 4.2 kpc. We measured the proper motion for WR 138a and found that it is a runaway star with a peculiar velocity of similar or equal to 50 km s-1. Implications of the runaway nature of WR 138a for constraining the mass of its progenitor star and understanding the origin of its ring nebula are discussed. Y1 - 2009 UR - http://www3.interscience.wiley.com/cgi-bin/issn?DESCRIPTOR=PRINTISSN&VALUE=0035-8711 U6 - https://doi.org/10.1111/j.1365-2966.2009.15492.x SN - 0035-8711 ER - TY - JOUR A1 - Gvaramadze, Vasily V. A1 - Kniazev, Alexei Y. A1 - Hamann, Wolf-Rainer A1 - Berdnikov, Leonid N. A1 - Fabrika, Sergei Nikolaevich A1 - Valeev, Azamat F. T1 - A new Wolf-Rayet star and its circumstellar nebula in Aquila N2 - We report the discovery of a new Wolf-Rayet star in Aquila via detection of its circumstellar nebula (reminiscent of ring nebulae associated with late WN stars) using the Spitzer Space Telescope archival data. Our spectroscopic follow-up of the central point source associated with the nebula showed that it is a WN7h star (we named it WR121b). We analysed the spectrum of WR 121b by using the Potsdam Wolf-Rayet model atmospheres, obtaining a stellar temperature of similar or equal to 50 kK. The stellar wind composition is dominated by helium with similar to 20 per cent of hydrogen. The stellar spectrum is highly reddened [E(B - V) = 2.85 mag]. Adopting an absolute magnitude of M-v = 5.7, the star has a luminosity of log L/L-circle dot = 5.75 and a mass-loss rate of 10(-4.7)M(circle dot)yr(-1), and resides at a distance of 6.3 kpc. We searched for a possible parent cluster of WR 121b and found that this star is located at similar or equal to 1 degrees from the young star cluster embedded in the giant HII region W43 (containing a WN7+a/OB? star - WR121a). We also discovered a bow shock around the O9.5III star ALS 9956, located at similar or equal to 0 degrees.5 from the cluster. We discuss the possibility that WR121b and ALS 9956 are runaway stars ejected from the cluster in W43. Y1 - 2010 UR - http://www3.interscience.wiley.com/cgi-bin/issn?DESCRIPTOR=PRINTISSN&VALUE=0035-8711 U6 - https://doi.org/10.1111/j.1365-2966.2009.16126.x SN - 0035-8711 ER - TY - JOUR A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Shenar, Tomer A1 - Marchant Campos, Pablo A1 - Eldridge, J. J. A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Langer, Norbert A1 - Todt, Helge Tobias T1 - Observational properties of massive black hole binary progenitors JF - Astronomy and astrophysics : an international weekly journal N2 - Context: The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Aims: Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Methods: Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. Results: The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Conclusions: While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed. KW - gravitational waves KW - binaries: close KW - stars: early-type KW - stars: atmospheres KW - stars: winds KW - outflows KW - stars: mass-loss Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731449 SN - 1432-0746 VL - 609 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Pasemann, Diana A1 - Todt, Helge Tobias A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer T1 - Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Aims. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. Methods. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. Results. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10(5.5) to 10(6.1) L-circle dot. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC. Conclusions. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past. KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: early-type KW - stars: atmospheres KW - stars: winds, outflows KW - stars: mass-loss Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201526241 SN - 1432-0746 VL - 581 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Ramachandran, Varsha A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Gruner, David A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - PoWR grids of non-LTE model atmospheres for OB-type stars of various metallicities JF - Astronomy and astrophysics : an international weekly journal N2 - The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website. KW - stars: massive KW - stars: early-type KW - stars: atmospheres KW - stars: winds KW - outflows KW - stars: mass-loss KW - radiative transfer Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201833787 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Ruehling, Ute A1 - Todt, Helge Tobias A1 - Oskinova, Lida A1 - Liermann, A. A1 - Graefener, G. A1 - Foellmi, C. A1 - Schnurr, O. A1 - Hamann, Wolf-Rainer T1 - The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10(6) L-circle dot and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L-circle dot) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L-circle dot) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L-circle dot) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M-circle dot. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation. KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: early-type KW - stars: atmospheres KW - stars: winds, outflows KW - stars: mass-loss Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201322696 SN - 0004-6361 SN - 1432-0746 VL - 565 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Rühling, U. A1 - Pasemann, D. A1 - Hamann, Wolf-Rainer T1 - The WN population in the Magellanic Clouds JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - A detailed and comprehensive study of the Wolf-Rayet stars of the nitrogen sequence (WN stars) in the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC) is presented. We derived the fundamental stellar and wind parameters for more than 100 massive stars, encompassing almost the whole WN population in the Magellanic Clouds (MCs). The observations are fitted with synthetic spectra, using the PotsdamWolf-Rayet model atmosphere code (PoWR). For this purpose, large grids of line-blanket models for different metallicities have been calculated, covering a wide range of stellar temperatures, mass-loss rates, and hydrogen abundances. Our comprehensive sample facilitates statistical studies of the WN properties in the MCs without selection bias. To investigate the impact of the low LMC metallicity and the even lower SMC metallicity, we compare our new results to previous analyses of the Galactic WN population and the late type WN stars from M31. Based on these studies we derived an empirical relation between the WN mass-loss rates and the metallicity. Current stellar evolution tracks, even when accounting for rotationally induced mixing, partly fail to reproduce the observed ranges of luminosities and initial masses. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-87806 SP - 117 EP - 120 ER - TY - JOUR A1 - Hajduk, Marcin A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Borek, Karolina A1 - van Hoof, Peter A. M. A1 - Zijlstra, Albert A. T1 - The cooling-down central star of the planetary nebula SwSt 1 BT - a late thermal pulse in a massive post-AGB star? JF - Monthly notices of the Royal Astronomical Society N2 - SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O III] 4959 and 5007 angstrom to H beta line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star. KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution KW - planetary KW - nebulae: general KW - planetary nebulae: individual: SwSt1 Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa2274 SN - 0035-8711 SN - 1365-2966 VL - 498 IS - 1 SP - 1205 EP - 1220 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hamann, Wolf-Rainer T1 - Basic ali in moving atmospheres N2 - The non-LTE radiative transfer problem requires the consistent solution of two sets of equations: the radiative transfer equations, which couple the spatial points, and the equations of the statistical equilibrium, which couple the frequencies. The "Accelerated Lambda Iteration" (ALI) method allows for an iterative scheme, in which both sets of equations are solved in turn. For moving atmospheres the radiative transfer is preferably formulated in the co-moving frame-of-reference, which leads to a partial differential equation. "Classical" numerical solution methods are based on differencing schemes. For better numerical stability, we prefer "short characteristics" integration methods. Iron line blanketing is accounted for by means of the "superlevel" concept. In contrast to static atmospheres, the frequencies can not be re-ordered in the moving case because of the frequency coupling from Doppler shifts. One of our future aims is the coupling of elaborated radiative transfer calculations with the hydrodynamical equations in order to understand the driving of strong stellar winds, especially from Wolf-Rayet stars. Y1 - 2003 SN - 1-5838-1131-1 ER - TY - JOUR A1 - Hamann, Wolf-Rainer T1 - Spectral analysis and model atmospheres of WR type central stars Y1 - 1996 ER - TY - JOUR A1 - Hamann, Wolf-Rainer T1 - Spectra of Wolf-Rayet type central stars and their analysis Y1 - 1997 ER - TY - JOUR A1 - Hamann, Wolf-Rainer T1 - Stellar winds from hot low-mass stars N2 - Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf-Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with "normal" composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. Y1 - 2010 UR - http://www.springerlink.com/content/100241 U6 - https://doi.org/10.1007/s10509-010-0344-8 SN - 0004-640X ER - TY - JOUR A1 - Hamann, Wolf-Rainer T1 - Wind models and spectral analyses JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - The emission-line dominated spectra of Wolf-Rayet stars are formed in expanding layers of their atmosphere, i.e. in their strong stellar wind. Adequate modeling of such spectra has to face a couple of difficulties. Because of the supersonic motion, the radiative transfer is preferably formulated in the co-moving frame. The strong deviations from local thermodynamical equilibrium (LTE) require to solve the equations of statistical equilibrium for the population numbers, accounting for many hundred atomic energy levels and thousands of line transitions. Moreover, millions of lines from iron-group elements must be taken into account for their blanketing effect. Model atmospheres of the described kind can reproduce the observed WR spectra satisfyingly, and have been widely applied for corresponding spectral analyses. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-87748 SP - 91 EP - 96 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Brown, John C. A1 - Feldmeier, Achim A1 - Oskinova, Lida T1 - On the wavelength drift of spectral features from structured hot star winds N2 - Spectral lines formed in stellar winds from OB stars are observed to exhibit profile variations. Discrete Absorption Components (DACs) show a remarkably slow wavelength drift with time. In a straightforward interpretation, this is in sharp contradiction to the steep velocity law predicted by the radiation-driven wind theory, and by semi- empirical profile fitting. In the present paper we re-discuss the interpretation of the drift rate. We show that the Co- rotating Interaction Region (CIR) model for the formation of DACs does not explain their slow drift rate as a consequence of rotation. On the contrary, the apparent acceleration of a spectral CIR feature is even higher than for the corresponding kinematical model without rotation. However, the observations can be understood by distinguishing between the velocity field of the matter flow, and the velocity law for the motion of the patterns in which the DAC features are formed. If the latter propagate upstream against the matter flow, the resulting wavelength drift mimics a much slower acceleration although the matter is moving fast. Additional to the DACs, a second type of recurrent structures is present in observed OB star spectra, the so-called modulations. In contrast to the DACs, these structures show a steep acceleration compatible with the theoretically predicted velocity law. We see only two possible consistent scenarios. Either, the wind is accelerated fast, and the modulations are formed in advected structures, while the DACs come from structures which are propagating upstream. Or, alternatively, steep and shallow velocity laws may co-exist at the same time in different spatial regions or directions of the wind. Y1 - 2001 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Graefener, G. A1 - Liermann, A. T1 - The galactic WN stars - Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. Aims. The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. Methods. We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. Results. We obtain a homogeneous set of stellar and atmospheric parameters for the GalacticWN stars, partly revising earlier results. Conclusions. Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood. KW - stars : mass-loss KW - stars : winds, outflows KW - stars : Wolf-Rayet KW - stars : atmospheres KW - stars : early-type KW - stars : evolution Y1 - 2006 U6 - https://doi.org/10.1051/0004-6361:20065052 SN - 0004-6361 VL - 457 IS - 3 SP - 1015 EP - 1031 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, G. A1 - Liermann, A. A1 - Hainich, Rainer A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Ramachandran, Varsha A1 - Todt, Helge Tobias A1 - Oskinova, Lida T1 - The Galactic WN stars revisited BT - Impact of Gaia distances on fundamental stellar parameters JF - Astronomy and astrophysics : an international weekly journal N2 - Comprehensive spectral analyses of the Galactic Wolf-Rayet stars of the nitrogen sequence (i.e. the WN subclass) have been performed in a previous paper. However, the distances of these objects were poorly known. Distances have a direct impact on the "absolute" parameters, such as luminosities and mass-loss rates. The recent Gaia Data Release (DR2) of trigonometric parallaxes includes nearly all WN stars of our Galactic sample. In the present paper, we apply the new distances to the previously analyzed Galactic WN stars and rescale the results accordingly. On this basis, we present a revised catalog of 55 Galactic WN stars with their stellar and wind parameters. The correlations between mass-loss rate and luminosity show a large scatter, for the hydrogen-free WN stars as well as for those with detectable hydrogen. The slopes of the log L - log M correlations are shallower than found previously. The empirical Hertzsprung-Russell diagram (HRD) still shows the previously established dichotomy between the hydrogen-free early WN subtypes that are located on the hot side of the zero-age main sequence (ZAMS), and the late WN subtypes, which show hydrogen and reside mostly at cooler temperatures than the ZAMS (with few exceptions). However, with the new distances, the distribution of stellar luminosities became more continuous than obtained previously. The hydrogen-showing stars of late WN subtype are still found to be typically more luminous than the hydrogen-free early subtypes, but there is a range of luminosities where both subclasses overlap. The empirical HRD of the Galactic single WN stars is compared with recent evolutionary tracks. Neither these single-star evolutionary models nor binary scenarios can provide a fully satisfactory explanation for the parameters of these objects and their location in the HRD. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - stars: atmospheres KW - stars: evolution KW - stars: distances Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834850 SN - 1432-0746 VL - 625 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - Grids of model spectra for WN stars, ready for use N2 - Grids of model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN subclass) are presented. The calculations account for the expansion of the atmosphere, non-LTE, clumping, and line blanketing from iron-group elements. Observed spectra of single Galactic WN stars can in general be reproduced consistently by this generation of models. The parameters of the presented model grids cover the whole relevant range of stellar temperatures and mass-loss rates. We point out that there is a degeneracy of parameters for very thick winds; their spectra tend to depend only on the ratio $L/{dot M}^{4/3}$. Abundances of the calculated grids are for Galactic WN stars without hydrogen and with 20% hydrogen (by mass), respectively. Model spectra and fluxes are available via internet (http://www.astro.physik.uni- potsdam.de/PoWR.html). Y1 - 2004 SN - 0004-6361 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - A temperature correction method for expanding atmospheres Y1 - 2004 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - The surface composition of hydrogen-deficient Post-AGB stars N2 - Most Central Stars of Planetary Nebulae exhibit a spectrum of a hydrogen-rich hot star with little or no stellar wind. About 20 % of the CSPN, however, show entirely different spectra dominated by bright and broad emission lines of carbon, oxygen and helium, resembling the so-called Wolf-Rayet (WR) spectral class originally established for massive, Pop. I stars. These spectra indicate a hydrogen-deficient surface composition and, at the same time, strong mass-loss. As the WR spectra are formed entirely in a dense stellar wind, their spectral analysis requires adequate modelling. Corresponding Non-LTE model atmospheres have been developed in the last decade and became more and more sophisticated. They have been applied yet for analyzing almost all available WR-type CSPN spectra, establishing the stellar parameters. The obtained surface abundances are not understandable in terms of "classical" evolutionary calculations, but agree in principle with the advanced models for AGB evolution which account consistently for diffusive mixing and nuclear burning. The underabundance of iron, which we established in a recent study of a WC-type central star (LMC-SMP 61), gives indirect evidence that neutron-capture synthesis has converted Fe into s-process elements. Y1 - 2003 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - The surface composition of hydrogen-deficient Post-AGB stars Y1 - 2003 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz A1 - Koesterke, Lars T1 - Wolf-Rayet star parameters from spectral analyses N2 - The Potsdam Non-LTE code for expanding atmospheres, which accounts for clumping and iron-line blanketing, has been used to establish a grid of model atmospheres for WC stars. A parameter degeneracy is discovered for early-type WC models which do not depend on the "stellar temperature". 15 galactic WC4-7 stars are analyzed, showing a very uniform carbon abundance (He:C=55:40) with only few exceptions. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz A1 - Koesterke, Lars T1 - WR Central Stars N2 - Wolf-Rayet type central stars have been analyzed with adequate model atmospheres. The obtained stellar parameters and chemical abundances allow for a discussion of their evolutionary origin. Y1 - 2003 SN - 1-583-81148-6 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Heber, Ulrich A1 - Jeffrey, C. S. T1 - Wolf-Rayet stars of high and low mass Y1 - 1996 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Jeffrey, C. S. A1 - Leuenhagen, U. T1 - Spectral analyses of late type central stars of planetary nebulae Y1 - 1996 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - Spectrum formation in clumped stellar winds : consequences for the analyses of Wolf-Rayet spectra Y1 - 1998 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - The nitrogen spectra of Wolf-Rayet stars Y1 - 1998 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - Spectral analyses with the standard model : Part II: Wolf-Rayet Stars Y1 - 1996 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - WN stars in the LMC : parameters and atmospheric abundances N2 - The spectra of 18 WN stars in the Large Magellanic Cloud (LMC) are quantitatively analyzed by means of "standard" Wolf-Rayet model atmospheres, using the helium and nitrogen lines as well as the spectral energy distribution. The hydrogen abundance is also determined. Carbon is included for a subset of 4 stars. The studied sample covers all spectral subtypes (WN2 ... WN9) and also includes one WN/WC transition object. The luminosities of the program stars span a wide range ( L/Lsun = 5.0 ... 6.5). Due to the given LMC membership, these results are free from uncertainties inferred from the distance. 50 % of the studied stars (both, late and early WN subtypes) have rather low luminosity (L/Lsun < 5.5). This puts tough constraints on their evolutionary formation. If coming from single stars, it provides evidence for strong internal mixing processes. The empirical mass-loss rates are scaled down by a factor of about two due to the impact of clumping, compared to previous studies adopting homogeneous winds. There is no obvious strong correlation between the mass-loss rates and other parameters like luminosity, temperature and composition. The stellar parameters for the present LMC sample are not systematically different from those of the Galactic WN stars studied previously with the same techniques, in contrast to the expected metallicity effects. Y1 - 2000 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - WM stars in the LMC : parameters and atmospheric abundances Y1 - 2000 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars T1 - Spectral analyses of Wolf-Rayet stars : the impact of clumping Y1 - 1999 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars A1 - Gräfener, Götz T1 - Spectral analyses of Wolf-Rayet winds N2 - The analysis of Wolf-Rayet spectra requires adequate model atmospheres which treat the non-LTE radiation transfer in a spherically expanding medium. Present state-of-the-art calculations account for complex model atoms with, typically, a few hundred energy levels and a few thousand spectral lines of He and CNO elements. In the most recent version of our model code, blanketing by millions of lines from iron-group elements is also included. These models have been widely applied for the spectral analysis of WN stars in the Galaxy and LMC. WN spectra can be well reproduced in most cases. WC stars have not yet been analyzed comprehensively, because the agreement with observations becomes satisfactory only when line-blanketed models are applied. The introduction of inhomogeneities (clumping), although treated in a rough approximation, has significantly improved the fit between synthetic and observed spectra with respect to the electron-scattering wings of strong lines. The mass-loss rates obtained from spectral analyses become smaller by a factor 2-3 if clumping is accounted for. A pre-specified velocity law is adopted for our models, but the radiation pressure can be evaluated from our detailed calculation and can be compared a posteriori with the required wind acceleration. Surprisingly we find that the line-blanketed models are not far from being hydrodynamically consistent, thus indicating that radiation pressure is probably the main driving force for the mass-loss from WR stars. Y1 - 2002 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars A1 - Gräfener, Götz T1 - Non-LTE models of WR winds Y1 - 2000 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars A1 - Gräfener, Götz T1 - Modelling and quantitative analyses of WR spectra : recent progress and results Y1 - 1999 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars A1 - Wesselowski, U. T1 - Spectral analyses of the galactic Wolf-Rayet stars : hydrogen-helium abundances and improved stellar parameters for the WN class Y1 - 1995 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Koesterke, Lars A1 - Wesselowski, U. T1 - Spectral atlas of galactic Wolf-Rayet stars (WN-sequence) Y1 - 1995 ER - TY - CHAP A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Feldmeier, Achim T1 - Spectrum formation in clumpy stellar winds N2 - Modeling expanding atmospheres is a difficult task because of the extreme non-LTE situation, the need to account for complex model atoms, especially for the iron-group elements with their millions of lines, and because of the supersonic expansion. Adequate codes have been developed e.g. by Hillier (CMFGEN), the Munich group (Puls, Pauldrach), and in Potsdam (PoWR code, Hamann et al.). While early work was based on the assumption of a smooth and homogeneous spherical stellar wind, the need to account for clumping became obvious about ten years ago. A relatively simple first-order clumping correction was readily implemented into the model codes. However, its simplifying assumptions are severe. Most importantly, the clumps are taken to be optically thin at all frequencies (”microclumping”). We discuss the consequences of this approximation and describe an approach to account for optically thick clumps (“macroclumping”). First results demonstrate that macroclumping can generally reduce the strength of spectral features, depending on their optical thickness. The recently reported discrepancy between the Hα diagnostic and the Pv resonance lines in O star spectra can be resolved without decreasing the mass-loss rates, when macroclumping is taken into account. Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-17838 ER - TY - THES A1 - Hamann, Wolf-Rainer A1 - Pena, M. A1 - Gräfener, Götz A1 - Ruiz, M. T. T1 - The central star of the planetary nebula N66 in the Large Magellanic Cloud : a detailed analysis of its dramatic evolution 1983 - 2000 Y1 - 2003 SN - 0004-6361 ER - TY - JOUR A1 - Hubrig, Swetlana A1 - Scholz, Kathleen A1 - Hamann, Wolf-Rainer A1 - Schoeller, M. A1 - Ignace, R. A1 - Ilyin, Ilya A1 - Gayley, K. G. A1 - Oskinova, Lida T1 - Searching for a magnetic field in Wolf-Rayet stars using FORS 2 spectropolarimetry JF - Monthly notices of the Royal Astronomical Society N2 - To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3 sigma (< B-z > = 258 +/- 78 G). Among the other targets, the highest value for the longitudinal magnetic field, < B-z > = 327 +/- 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the < B-z > variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights. KW - techniques: polarimetric KW - stars: individual: WR 6 KW - stars: magnetic field KW - stars: variables: general KW - stars: Wolf-Rayet Y1 - 2016 U6 - https://doi.org/10.1093/mnras/stw558 SN - 0035-8711 SN - 1365-2966 VL - 458 SP - 3381 EP - 3393 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Huenemoerder, D. A1 - Gayley, K. A1 - Hamann, Wolf-Rainer A1 - Ignace, R. A1 - Nichols, J. A1 - Oskinova, Lida A1 - Pollock, A. M. T. A1 - Schulz, N. T1 - High Resolution X-Ray Spectra of WR 6 JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - As WR 6 is a putatively single WN4 star, and is relatively bright (V = 6.9), it is an ideal case for studying the wind mechanisms in these extremely luminous stars. To obtain higher resolution spectra at higher energy (above 1 keV) than previously obtained with the XMM/Newton RGS, we have observed WR 6 with the Chandra High Energy Transmission Grating Spectrometer for 450 ks. We have resolved emission lines of S, Si, Mg, Ne, and Fe, which all show a “fin"-shaped prole, characteristic of a self-absorbed uniformly expanding shell. Steep blue edges gives robust maximal expansion velocities of about 2000 km/s, somewhat larger than the 1700km/s derived from UV lines. The He-like lines all indicate that X-ray emitting plasmas are far from the photosphere – even at the higher energies where opacity is lowest { as was also the case for the longer wavelength lines observed with XMM-Newton/RGS. Abundances determined from X-ray spectral modeling indicate enhancements consistent with nucleosynthesis. The star was also variable in X-rays and in simultaneous optical photometry obtained with Chandra aspect camera, but not coherently with the optically known period of 3.765 days. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88236 SP - 301 EP - 304 ER - TY - JOUR A1 - Huenemoerder, David P. A1 - Gayley, K. G. A1 - Hamann, Wolf-Rainer A1 - Ignace, R. A1 - Nichols, J. S. A1 - Oskinova, Lida A1 - Pollock, A. M. T. A1 - Schulz, Norbert S. A1 - Shenar, Tomer T1 - Probing Wolf-Rayet winds: Chandra/HETG X-ray spectra of WR 6 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear-processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars. KW - stars: individual (WR 6) KW - stars: massive KW - stars: Wolf-Rayet Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/815/1/29 SN - 0004-637X SN - 1538-4357 VL - 815 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Ignace, Rico A1 - Gayley, Kenneth G. A1 - Hamann, Wolf-Rainer A1 - Huenemoerder, David P. A1 - Oskinova, Lida A1 - Pollock, Andy M. T. A1 - McFall, Michael T1 - THE XMM-NEWTON/EPIC X-RAY LIGHT CURVE ANALYSIS OF WR 6 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We obtained four pointings of over 100 ks each of the well-studied Wolf-Rayet star WR 6 with the XMM-Newton satellite. With a first paper emphasizing the results of spectral analysis, this follow-up highlights the X-ray variability clearly detected in all four pointings. However, phased light curves fail to confirm obvious cyclic behavior on the well-established 3.766 day period widely found at longer wavelengths. The data are of such quality that we were able to conduct a search for event clustering in the arrival times of X-ray photons. However, we fail to detect any such clustering. One possibility is that X-rays are generated in a stationary shock structure. In this context we favor a corotating interaction region (CIR) and present a phenomenological model for X-rays from a CIR structure. We show that a CIR has the potential to account simultaneously for the X-ray variability and constraints provided by the spectral analysis. Ultimately, the viability of the CIR model will require both intermittent long-term X-ray monitoring of WR 6 and better physical models of CIR X-ray production at large radii in stellar winds. KW - stars: individual (WR 6) KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - X-rays: stars Y1 - 2013 U6 - https://doi.org/10.1088/0004-637X/775/1/29 SN - 0004-637X VL - 775 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - [WC]-type CSPN : clumping and wind-driving N2 - Many Central Stars of Planetary Nebulae are very similar to massive Wolf-Rayet stars of the carbon sequence with respect to their spectra, chemical composition and wind properties. Therefore their study opens an additional way towards the understanding of the Wolf-Rayet phenomenon. While the study of Line Profile Variation will be difficult, espescially for the very compact early types, the comparision with other hydrogen-deficient Central Stars illuminates the driving mechanism of their winds. We speculate that at least two ingredients are needed. The ionization of their atmpospheres has to be stratified to enable multi-scattering processes and the amount of carbon and oxygen has to be high (more than a few percent by mass). Y1 - 2002 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - Spectral analyses of central of planetary nebulae of early WC-type / NGC 6751 and Sanduleak 3 Y1 - 1997 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - Quantitative spectral analyses of CSPNs of early [WC]-type Y1 - 1997 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer T1 - Spectral analyses of 25 galactic Wolf-Rayet stars of the carbon sequence Y1 - 1995 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - Expanding atmospheres in non-LTE : Radiation transfer using short characteristics N2 - We present our technique for solving the equations of radiation transfer in spherically expanding atmospheres. To ensure an efficient treatment of the Thomson scattering, the mean intensity J is derived by solving the moment equations in turn with the angle-dependent transfer equation. The latter provide the Eddington factors. Two different methods for the solution of the angle dependent equation are compared. Thereby the integration along short characteristics turned out to be superior in our context over the classical differencing scheme. The method is the basis of a non-LTE code suitable for the atmospheres of hot stars with high mass-loss. Y1 - 2002 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - Inhomogeneities in Wolf-Rayet atmospheres Y1 - 1999 ER - TY - JOUR A1 - Koesterke, Lars A1 - Hamann, Wolf-Rainer A1 - Urrutia, Tanya T1 - Line-Profile Variability in the Wolf-Rayet Stars WR 135 and WR 111 N2 - We have obtained time-resolved observations of line-profile variations of the two Wolf-Rayet stars WR 135 and WR 111. The spectra, taken during two consecutive nights, cover a broad range from 4470 to 6590 Ang. The profile variability of the C iii emission line at 5696 Ang in WR 135 is shown in detail. The principal difficulties to constrain the velocity law from the frequency drift of discrete spectral features is discussed, emphasizing the crucial dependence on the adopted location of the line-emission region, and the possible necessity to distinguish between the motion of structures and the flow of the matter. - Full access to the observational data is provided via anonymous file transfer. Y1 - 2001 ER - TY - GEN A1 - Kubatova, B. A1 - Kubát, Jiří A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida T1 - Clumping in Massive Star Winds and its Possible Connection to the B[e] Phenomenon T2 - The B(e) Phenomenon: Forty Years of Studies : proceedings of a conference held at Charles University, Prague, Czech Republic, 27 June-1 July 2016 N2 - It has been observationally established that winds of hot massive stars have highly variable characteristics. The variability evident in the winds is believed to be caused by structures on a broad range of spatial scales. Small-scale structures (clumping) in stellar winds of hot stars are possible consequence of an instability appearing in their radiation hydrodynamics. To understand how clumping may influence calculation of theoretical spectra, different clumping properties and their 3D nature have to be taken into account. Properties of clumping have been examined using our 3D radiative transfer calculations. Effects of clumping for the case of the B[e] phenomenon are discussed. Y1 - 2017 SN - 978-1-58381-900-5 SN - 978-1-58381-901-2 VL - 508 SP - 45 EP - 50 PB - Astronomical Soceity of the Pacific CY - San Fransisco ER - TY - GEN A1 - Kubatova, Brankica A1 - Hamann, Wolf-Rainer A1 - Kubat, Jiri A1 - Oskinova, Lida T1 - 3D Monte Carlo Radiative Transfer in Inhomogeneous Massive Star Winds BT - Application to Resonance Line Formation T2 - Radiative signatures from the cosmos N2 - Already for decades it has been known that the winds of massive stars are inhomogeneous (i.e. clumped). To properly model observed spectra of massive star winds it is necessary to incorporate the 3-D nature of clumping into radiative transfer calculations. In this paper we present our full 3-D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. We use a set of parameters to describe dense as well as the rarefied wind components. At the same time, we account for non-monotonic velocity fields. We show how the 3-D density and velocity wind inhomogeneities strongly affect the resonance line formation. We also show how wind clumping can solve the discrepancy between P v and H alpha mass-loss rate diagnostics. Y1 - 2019 SN - 978-1-58381-925-8 SN - 1050-3390 VL - 519 SP - 209 EP - 212 PB - Astronomical soc pacific CY - San Fransisco ER - TY - JOUR A1 - Kubatova, Brankica A1 - Szecsi, D. A1 - Sander, Andreas Alexander Christoph A1 - Kubat, Jiří A1 - Tramper, F. A1 - Krticka, Jiri A1 - Kehrig, C. A1 - Hamann, Wolf-Rainer A1 - Hainich, Rainer A1 - Shenar, Tomer T1 - Low-metallicity massive single stars with rotation BT - II. Predicting spectra and spectral classes of chemically homogeneously evolving stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z(circle dot)) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20 130 M-circle dot) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime. KW - stars: massive KW - stars: winds, outflows KW - stars: rotation KW - galaxies: dwarf KW - radiative transfer Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834360 SN - 1432-0746 SN - 0004-6361 VL - 623 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Kubátová, Brankica A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias A1 - Sander, A. A1 - Steinke, M. A1 - Hainich, Rainer A1 - Shenar, Tomer T1 - Macroclumping in WR 136 JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - Macroclumping proved to resolve the discordance between different mass-loss rate diagnostics for O-type stars, in particular between Hα and the P v resonance lines. In this paper, we report first results from a corresponding investigation for WR stars. We apply our detailed 3-D Monte Carlo (MC) line formation code to the P v resonance doublet and show, for the Galactic WNL star WR136, that macroclumping is require to bring this line in accordance with the mass-loss rate derived from the emission-line spectrum. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-87823 SP - 125 EP - 128 ER - TY - JOUR A1 - Leuenhagen, U. A1 - Hamann, Wolf-Rainer T1 - Spectral analyses of late-type [WC] central stars of planetary nebulae : more empirical constraints for their evolutionary status Y1 - 1998 ER - TY - JOUR A1 - Liermann, A. A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida T1 - The Quintuplet cluster III. Hertzsprung-Russell diagram and cluster age JF - Astronomy and astrophysics : an international weekly journal N2 - The Quintuplet, one of three massive stellar clusters in the Galactic center (GC), is located about 30 pc in projection from Sagittarius A*. We aim at the construction of the Hertzsprung-Russell diagram (HRD) of the cluster to study its evolution and to constrain its star-formation history. For this purpose we use the most complete spectral catalog of the Quintuplet stars. Based on the K-band spectra we determine stellar temperatures and luminosities for all stars in the catalog under the assumption of a uniform reddening towards the cluster. We find two groups in the resulting HRD: early-type OB stars and late-type KM stars, well separated from each other. By comparison with Geneva stellar evolution models we derive initial masses exceeding 8 M-circle dot for the OB stars. In the HRD these stars are located along an isochrone corresponding to an age of about 4 Myr. This confirms previous considerations, where a similar age estimate was based on the presence of evolved Wolf-Rayet stars in the cluster. We derive number ratios for the various spectral subtype groups (e.g. N-WR/N-O, N-WC/N-WN) and compare them with predictions of population synthesis models. We find that an instantaneous burst of star formation at about 3.3 to 3.6 Myr ago is the most likely scenario to form the Quintuplet cluster. Furthermore, we apply a mass-luminosity relation to construct the initial mass function (IMF) of the cluster. We find indications for a slightly top-heavy IMF. The late-type stars in the LHO catalog are red giant branch (RGB) stars or red supergiants (RSGs) according to their spectral signatures. Under the assumption that they are located at about the distance of the Galactic center we can derive their luminosities. The comparison with stellar evolution models reveals that the initial masses of these stars are lower than 15 M-circle dot implying that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to evolve into their present stage. It might be suspected that these late-type stars do not physically belong to the Quintuplet cluster. Indeed, most of them disqualify as cluster members because their radial velocities differ too much from the cluster average. Nevertheless, five of the brightest RGB/RSG stars from the LHO catalog share the mean radial velocity of the Quintuplet, and thus remain highly suspect for being gravitationally bound members. If so, this would challenge the cluster formation and evolution scenario. KW - stars: late-type KW - Hertzsprung-Russell and C-M diagrams KW - infrared: stars KW - stars: early-type KW - open cluster and associations: individual: Quintuplet Y1 - 2012 U6 - https://doi.org/10.1051/0004-6361/201117534 SN - 0004-6361 VL - 540 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Liermann, Angelika A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida T1 - The quintuplet cluster III. Hertzsprung-Russell diagram and cluster age (vol 540, pg A14, 2012) T2 - Astronomy and astrophysics : an international weekly journal KW - open clusters and associations: individual: Quintuplet KW - infrared: stars KW - stars: early-type KW - stars: late-type KW - Hertzsprung-Russell and C-M diagrams KW - errata, addenda Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201117534e SN - 0004-6361 SN - 1432-0746 VL - 563 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Martinez-Nunez, Silvia A1 - Sander, Angelika A1 - Gimenez-Garcia, Angel A1 - Gonzalez-Galan, Ana A1 - Torrejon, Jose Miguel A1 - Gonzalez-Fernandez, Carlos A1 - Hamann, Wolf-Rainer T1 - The donor star of the X-ray pulsar X1908+075 JF - Astronomy and astrophysics : an international weekly journal N2 - High-mass X-ray binaries consist of a massive donor star and a compact object. While several of those systems have been well studied in X-rays, little is known for most of the donor stars as they are often heavily obscured in the optical and ultraviolet regime. There is an opportunity to observe them at infrared wavelengths, however. The goal of this study is to obtain the stellar and wind parameters of the donor star in the X1908+075 high-mass X-ray binary system with a stellar atmosphere model to check whether previous studies from X-ray observations and spectral morphology lead to a sufficient description of the donor star. We obtained H-and K-band spectra of X1908+075 and analysed them with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. For the first time, we calculated a stellar atmosphere model for the donor star, whose main parameters are: M-spec = 15 +/- 6 M-circle dot, T-* = 23(-3)(+6) kK, log g(eff) = 3.0 +/- 0.2 and log L/L-circle dot = 4.81 +/- 0.25. The obtained parameters point towards an early B-type (B0-B3) star, probably in a supergiant phase. Moreover we determined a more accurate distance to the system of 4.85 +/- 0.50 kpc than the previously reported value. KW - binaries: close KW - stars: individual: X1908+075 KW - stars: massive KW - stars: winds KW - outflows KW - X-rays: binaries Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201424823 SN - 0004-6361 SN - 1432-0746 VL - 578 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Nichols, Joy A1 - Huenemoerder, David P. A1 - Corcoran, Michael F. A1 - Waldron, Wayne A1 - Naze, Yael A1 - Pollock, Andy M. T. A1 - Moffat, Anthony F. J. A1 - Lauer, Jennifer A1 - Shenar, Tomer A1 - Russell, Christopher M. P. A1 - Richardson, Noel D. A1 - Pablo, Herbert A1 - Evans, Nancy Remage A1 - Hamaguchi, Kenji A1 - Gull, Theodore A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Ignace, Rosina A1 - Hoffman, Jennifer L. A1 - Hole, Karen Tabetha A1 - Lomax, Jamie R. T1 - A COORDINATED X-RAY AND OPTICAL CAMPAIGN OF THE NEAREST MASSIVE ECLIPSING BINARY, delta ORIONIS Aa. II. X-RAY VARIABILITY JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present time-resolved and phase-resolved variability studies of an extensive X-ray high-resolution spectral data set of the delta Ori Aa binary system. The four observations, obtained with Chandra ACIS HETGS, have a total exposure time of approximate to 479 ks and provide nearly complete binary phase coverage. Variability of the total X-ray flux in the range of 5-25 is is confirmed, with a maximum amplitude of about +/- 15% within a single approximate to 125 ks observation. Periods of 4.76 and 2.04 days are found in the total X-ray flux, as well as an apparent overall increase in the flux level throughout the nine-day observational campaign. Using 40 ks contiguous spectra derived from the original observations, we investigate the variability of emission line parameters and ratios. Several emission lines are shown to be variable, including S XV, Si XIII, and Ne IX. For the first time, variations of the X-ray emission line widths as a function of the binary phase are found in a binary system, with the smallest widths at phi = 0.0 when the secondary delta Ori Aa2 is at the inferior conjunction. Using 3D hydrodynamic modeling of the interacting winds, we relate the emission line width variability to the presence of a wind cavity created by a wind-wind collision, which is effectively void of embedded wind shocks and is carved out of the X-ray-producing primary wind, thus producing phase-locked X-ray variability. KW - binaries: close KW - binaries: eclipsing KW - stars: individual ([HD 36486]delta Ori A) Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/809/2/133 SN - 0004-637X SN - 1538-4357 VL - 809 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - X-ray emission lines from inhomogeneous stellar winds N2 - It is commonly adopted that X-rays from O stars are produced deep inside the stellar wind, and transported outwards through the bulk of the expanding matter which attenuates the radiation and affects the shape of emission line profiles. The ability of the X-ray observatories Chandra and XMM-Newton to resolve these lines spectroscopically provided a stringent test for the theory of the X-ray production. It turned out that none of the existing models was able to fit the observations consistently. The possible caveat of these models was the underlying assumption of a smooth stellar wind. Motivated by the evidence that the stellar winds are in fact structured, we present a 2-D numerical model of a stochastic, inhomogeneous wind. Small parcels of hot, X-ray emitting gas are permeated by cool, absorbing wind material which is compressed into thin shell fragments. Wind fragmentation alters the radiative transfer drastically, compared to homogeneous models of the same mass-loss rate. X-rays produced deep inside the wind, which would be totally absorbed in a homogeneous flow, can effectively escape from a fragmented wind. The wind absorption becomes wavelength independent if the individual fragments are optically thick. The X-ray line profiles are flat-topped in the blue part and decline steeply in the red part for the winds with a short acceleration zone. For the winds where the acceleration extends over significant distances, the lines can appear nearly symmetric and only slightly blueshifted, in contrast to the skewed, triangular line profiles typically obtained from homogeneous wind models of high optical depth. We show that profiles from a fragmented wind model can reproduce the observed line profiles from zeta Orionis. The present numerical modeling confirms the results from a previous study, where we derived analytical formulae from a statistical treatment Y1 - 2004 SN - 0004-6361 ER - TY - JOUR A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - X-ray line profiles from structured stellar winds N2 - Absorbing material compressed in a number of thin shells is effectively less opaque for X-rays than smoothly distributed gas. The calculated X-ray emission line profiles are red-shifted if the emission arises from the starward side of the shells. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - High-resolution X-ray spectroscopy of bright O-type stars JF - Monthly notices of the Royal Astronomical Society N2 - Archival X-ray spectra of the four prominent single, non-magnetic O stars zeta Pup, zeta Ori, xi Per and zeta Oph, obtained in high resolution with Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles provide information about the shocked, hot gas which emits the X-radiation, and about the bulk of comparably cool stellar wind material which partly absorbs this radiation. In this paper, we synthesize X-ray line profiles with a model of a clumpy stellar wind. We find that the geometrical shape of the wind inhomogeneities is important: better agreement with the observations can be achieved with radially compressed clumps than with spherical clumps. The parameters of the model, i.e. chemical abundances, stellar radius, mass-loss rate and terminal wind velocity, are taken from existing analyses of UV and optical spectra of the programme stars. On this basis, we also calculate the continuum-absorption coefficient of the cool-wind material, using the Potsdam Wolf-Rayet (POWR) model atmosphere code. The radial location of X-ray emitting gas is restricted from analysing the FIR line ratios of helium-like ions. The only remaining free parameter of our model is the typical distance between the clumps; here, we assume that at any point in the wind there is one clump passing by per one dynamical time-scale of the wind. The total emission in a model line is scaled to the observation. There is a good agreement between synthetic and observed line profiles. We conclude that the X-ray emission line profiles in O stars can be explained by hot plasma embedded in a cool wind which is highly clumped in the form of radially compressed shell fragments. KW - stars : individual : zeta Pup KW - stars : individual : zeta Ori KW - stars : individual : xi Per KW - stars : individual : zeta Oph KW - X-rays : stars Y1 - 2006 U6 - https://doi.org/10.1111/j.1365-2966.2006.10858.x SN - 0035-8711 VL - 372 SP - 313 EP - 326 PB - Oxford University Press CY - Oxford ER - TY - GEN A1 - Oskinova, Lida A1 - Gayley, K. G. A1 - Hamann, Wolf-Rainer A1 - Hünemörder, D. P. A1 - Ignace, R. A1 - Pollock, A. M. T. T1 - High-Resolution X-Ray Spectroscopy reveals the special nature of Wolf-Rayet star winds (pg 747, 2012) T2 - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters Y1 - 2012 U6 - https://doi.org/10.1088/2041-8205/752/2/L35 SN - 2041-8205 VL - 752 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Cassinelli, Joseph P. A1 - Brown, John C. A1 - Todt, Helge Tobias T1 - X-ray emission from massive stars with magnetic fields JF - Astronomische Nachrichten = Astronomical notes N2 - We investigate the connections between the magnetic fields and the X-ray emission from massive stars. Our study shows that the X-ray properties of known strongly magnetic stars are diverse: while some comply to the predictions of the magnetically confined wind model, others do not. We conclude that strong, hard, and variable X-ray emission may be a sufficient attribute of magnetic massive stars, but it is not a necessary one. We address the general properties of X-ray emission from "normal" massive stars, especially the long standing mystery about the correlations between the parameters of X-ray emission and fundamental stellar properties. The recent development in stellar structure modeling shows that small-scale surface magnetic fields may be common. We suggest a "hybrid" scenario which could explain the X-ray emission from massive stars by a combination of magnetic mechanisms on the surface and shocks in the stellar wind. The magnetic mechanisms and the wind shocks are triggered by convective motions in sub-photospheric layers. This scenario opens the door for a natural explanation of the well established correlation between bolometric and X-ray luminosities. KW - stars: magnetic fields KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - techniques: spectroscopic KW - X-rays: stars Y1 - 2011 U6 - https://doi.org/10.1002/asna.201111602 SN - 0004-6337 VL - 332 IS - 9-10 SP - 988 EP - 993 PB - Wiley-Blackwell CY - Malden ER - TY - CHAP A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Feldmeier, Achim T1 - X-raying clumped stellar winds N2 - X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. Generally, to correctly model the emerging Xray spectra, a detailed knowledge of the cool wind opacities based on stellar atmosphere models is prerequisite. A nearly grey stellar wind opacity for the X-rays is deduced from the analyses of high-resolution X-ray spectra. This indicates that the stellar winds are strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission line profiles can be explained if the wind clumps are radially compressed. In massive binaries the orbital variations of X-ray emission allow to probe the opacity of the stellar wind; results support the picture of strong wind clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the extend of the stellar-wind part photoionized by X-rays provide further strong evidence that stellar winds consist of dense clumps. Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-18133 ER - TY - JOUR A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Feldmeier, Achim A1 - Ignace, Richard A1 - Chu, You-Hua T1 - Discovery of X-ray emission from the Wolf-Rayet star WR 142 of oxygen subtype N2 - We report the discovery of weak yet hard X-ray emission from the Wolf-Rayet (WR) star WR 142 with the XMM- Newton X-ray telescope. Being of spectral subtype WO2, WR 142 is a massive star in a very advanced evolutionary stage shortly before its explosion as a supernova or. gamma-ray burst. This is the first detection of X-ray emission from a WO- type star. We rule out any serendipitous X-ray sources within approximate to 1 '' of WR 142. WR 142 has an X- ray luminosity of L-X approximate to 7 x 10(30) erg s(-1), which constitutes only less than or similar to 10(-8) of its bolometric luminosity. The hard X-ray spectrum suggests a plasma temperature of about 100 MK. Commonly, X-ray emission from stellar winds is attributed to embedded shocks due to the intrinsic instability of the radiation driving. From qualitative considerations we conclude that this mechanism cannot account for the hardness of the observed radiation. There are no hints for a binary companion. Therefore the only remaining, albeit speculative explanation must refer to magnetic activity. Possibly related, WR 142 seems to rotate extremely fast, as indicated by the unusually round profiles of its optical emission lines. Our detection implies that the wind of WR 142 must be relatively transparent to X-rays, which can be due to strong wind ionization, wind clumping, or nonspherical geometry from rapid rotation. Y1 - 2009 UR - http://iopscience.iop.org/0004-637X/ U6 - https://doi.org/10.1088/0004-637x/693/1/L44 SN - 0004-637X ER - TY - JOUR A1 - Oskinova, Lida A1 - Huenemoerder, D. P. A1 - Hamann, Wolf-Rainer A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Ignace, R. A1 - Todt, Helge Tobias A1 - Hainich, Rainer T1 - On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions. KW - stars: individual (Cyg OB2 12) KW - stars: massive KW - stars: mass-loss KW - stars: winds, outflows KW - supergiants KW - X-rays: stars Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa7e79 SN - 0004-637X SN - 1538-4357 VL - 845 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Oskinova, Lida A1 - Ignace, Richard A1 - Hamann, Wolf-Rainer A1 - Pollock, A. M. T. A1 - Brown, John C. T1 - The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars N2 - The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of Lx < 2.5 x 1030 ergs-1 and to the X-ray to bolometric luminosity ratio of Lx/Lbol < 4 x 10-9. This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and CHANDRA observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven stellar winds of this particular class are insignificant X-ray sources. We attribute this to photoelectronic absorption by the stellar wind. The high opacity of the metal-rich and dense winds from WC stars puts the radius of optical depth unity at hundreds or thousands of stellar radii for much of the X-ray band. We believe that the essential absence of hot plasma so far out in the wind exacerbated by the large distances and correspondingly high ISM column densities makes the WC stars too faint to be detectable with current technology. The result also applies to many WC stars in binary systems, of which only about 20 % are identified X-ray sources, presumably due to colliding winds. Y1 - 2003 ER - TY - JOUR A1 - Oskinova, Lida A1 - Kubatova, Brankica A1 - Hamann, Wolf-Rainer T1 - Moving inhomogeneous envelopes of stars JF - Transport in Porous Media N2 - Massive stars are extremely luminous and drive strong winds, blowing a large part of their matter into the galactic environment before they finally explode as a supernova. Quantitative knowledge of massive star feedback is required to understand our Universe as we see it. Traditionally, massive stars have been studied under the assumption that their winds are homogeneous and stationary, largely relying on the Sobolev approximation. However, Observations with the newest instruments, together with progress in model calculations, ultimately dictate a cardinal change of this paradigm: stellar winds are highly inhomogeneous. Hence, we are now advancing to a new stage in our understanding of stellar winds. Using the foundations laid by V.V. Sobolev and his school, we now update and further develop the stellar spectral analysis techniques. New sophisticated 3-D models of radiation transfer in inhomogeneous expanding media elucidate the physics of stellar winds and improve classical empiric mass-loss rate diagnostics. Applications of these new techniques to multiwavelength observations of massive stars yield consistent and robust stellar wind parameters. (C) 2016 Elsevier Ltd. All rights reserved. KW - Stars: mass-loss KW - Stars: winds KW - Outflows KW - Stars: atmospheres early type Y1 - 2016 U6 - https://doi.org/10.1016/j.jqsrt.2016.06.017 SN - 0022-4073 SN - 1879-1352 VL - 183 SP - 100 EP - 112 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Oskinova, Lida A1 - Steinke, M. A1 - Hamann, Wolf-Rainer A1 - Sander, A. A1 - Todt, Helge Tobias A1 - Liermann, Adriane T1 - One of the most massive stars in the Galaxy may have formed in isolation JF - Monthly notices of the Royal Astronomical Society N2 - Very massive stars, 100 times heavier than the sun, are rare. It is not yet known whether such stars can form in isolation or only in star clusters. The answer to this question is of fundamental importance. The central region of our Galaxy is ideal for investigating very massive stars and clusters located in the same environment. We used archival infrared images to investigate the surroundings of apparently isolated massive stars presently known in the Galactic Centre (GC). We find that two such isolated massive stars display bow shocks and hence may be 'runaways' from their birthplace. Thus, some isolated massive stars in the GC region might have been born in star clusters known in this region. However, no bow shock is detected around the isolated star WR 102ka (Peony nebula star), which is one of the most massive and luminous stars in the Galaxy. This star is located at the centre of an associated circumstellar nebula. To study whether a star cluster may be 'hidden' in the surroundings of WR 102ka, to obtain new and better spectra of this star, and to measure its radial velocity, we obtained observations with the integral-field spectrograph SINFONI at the ESO's Very Large Telescope. Our observations confirm that WR 102ka is one of the most massive stars in the Galaxy and reveal that this star is not associated with a star cluster. We suggest that WR 102ka has been born in relative isolation, outside of any massive star cluster. KW - stars: individual: WR 102ka KW - Galaxy: centre KW - infrared: stars Y1 - 2013 U6 - https://doi.org/10.1093/mnras/stt1817 SN - 0035-8711 SN - 1365-2966 VL - 436 IS - 4 SP - 3357 EP - 3365 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Oskinova, Lida A1 - Naze, Yael A1 - Todt, Helge Tobias A1 - Huenemoerder, David P. A1 - Ignace, Richard A1 - Hubrig, Swetlana A1 - Hamann, Wolf-Rainer T1 - Discovery of X-ray pulsations from a massive star JF - Nature Communications N2 - X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. All X-ray pulsars known so far are associated with degenerate objects, either neutron stars or white dwarfs. Here we report the discovery of pulsating X-rays from a non-degenerate object, the massive B-type star xi(1) CMa. This star is a variable of beta Cep-type and has a strong magnetic field. Our observations with the X-ray Multi-Mirror (XMM-Newton) telescope reveal X-ray pulsations with the same period as the fundamental stellar oscillations. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission and their magnetism. Y1 - 2014 U6 - https://doi.org/10.1038/ncomms5024 SN - 2041-1723 VL - 5 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Oskinova, Lida A1 - Todt, Helge Tobias A1 - Huenemoerder, David P. A1 - Hubrig, Swetlana A1 - Ignace, Richard A1 - Hamann, Wolf-Rainer A1 - Balona, Luis T1 - On X-ray pulsations in beta Cephei-type variables JF - Astronomy and astrophysics : an international weekly journal N2 - Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays. Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars. Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity. Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question. KW - stars: massive KW - stars: variables: general KW - X-rays: stars Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201525908 SN - 0004-6361 SN - 1432-0746 VL - 577 PB - EDP Sciences CY - Les Ulis ER -