@article{TorrejonSchulzNowaketal.2015, author = {Torrejon, Jose M. and Schulz, Norbert S. and Nowak, Michael A. and Oskinova, Lida and Rodes-Roca, Jose J. and Shenar, Tomer and Wilms, J{\"o}rn}, title = {On the radial onset of clumping in the wind of the B0I massive star QV nor}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {810}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/810/2/102}, pages = {11}, year = {2015}, abstract = {We present an analysis of a 78 ks Chandra high-energy transmission gratings observation of the B0I star QV Nor, the massive donor of the wind-accreting pulsar 4U1538-52. The neutron star (NS) orbits its companion in a very close orbit (r < 1.4R(*), in units of the stellar radii), thereby allowing probing of the innermost wind regions. The flux of the Fe K alpha line during eclipse reduces to only similar to 30\% of the flux measured out of eclipse. This indicates that the majority of Fe fluorescence must be produced in regions close to the NS, at distances smaller than 1R(*) from its surface. The fact that the flux of the continuum decreases to only similar to 3\% during eclipse allows for a high contrast of the Fe Ka line fluorescence during eclipse. The line is not resolved and centered at lambda = 1.9368(-0.0018)(+0.0032) angstrom. From the inferred plasma speed limit of v < c Delta lambda/lambda < 800 km s(-1) and range of ionization parameters of log xi =[-1, 2], together with the stellar density profile, we constrain the location of the cold, dense material in the stellar wind of QV Nor using simple geometrical considerations. We then use the Fe K alpha line fluorescence as a tracer of wind clumps and determine that these clumps in the stellar wind of QV Nor (B0I) must already be present at radii r < 1.25R(*), close to the photosphere of the star.}, language = {en} } @article{TodtSanderHainichetal.2015, author = {Todt, Helge Tobias and Sander, Angelika and Hainich, Rainer and Hamann, Wolf-Rainer and Quade, Markus and Shenar, Tomer}, title = {Potsdam Wolf-Rayet model atmosphere grids for WN stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {579}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201526253}, pages = {6}, year = {2015}, abstract = {We present new grids of Potsdam Wolf-Rayet (PoWR) model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN stars). The models have been calculated with the latest version of the PoWR stellar atmosphere code for spherical stellar winds. The WN model atmospheres include the non-LTE solutions of the statistical equations for complex model atoms, as well as the radiative transfer equation in the co-moving frame. Iron-line blanketing is treated with the help of the superlevel approach, while wind inhomogeneities are taken into account via optically thin clumps. Three of our model grids are appropriate for Galactic metallicity. The hydrogen mass fraction of these grids is 50\%, 20\%, and 0\%, thus also covering the hydrogen-rich late-type WR stars that have been discovered in recent years. Three grids are adequate for LMC WN stars and have hydrogen fractions of 40\%, 20\%, and 0\%. Recently, additional grids with SMC metallicity and with 60\%, 40\%, 20\%, and 0\% hydrogen have been added. We provide contour plots of the equivalent widths of spectral lines that are usually used for classification and diagnostics.}, language = {en} } @article{ToalaRamosLariosGuerreroetal.2019, author = {Toal{\´a}, Jes{\´u}s Alberto and Ramos-Larios, Gerardo and Guerrero, Martin A. and Todt, Helge Tobias}, title = {Hidden IR structures in NGC40}, series = {Monthly notices of the Royal Astronomical Society}, volume = {485}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stz624}, pages = {3360 -- 3369}, year = {2019}, abstract = {We present the analysis of infrared (IR) observations of the planetary nebula NGC40 together with spectral analysis of its [WC]-type central starHD826. Spitzer IRS observations were used to produce spectral maps centred at polycyclic aromatic hydrocarbons (PAH) bands and ionic transitions to compare their spatial distribution. The ionic lines show a clumpy distribution of material around the main cavity of NGC40, with the emission from [Ar II] being the most extended, whilst the PAHs show a rather smooth spatial distribution. Analysis of ratio maps shows the presence of a toroidal structure mainly seen in PAH emission, but also detected in a Herschel PACS 70 mu m image. We argue that the toroidal structure absorbs the UV flux from HD826, preventing the nebula to exhibit lines of high-excitation levels as suggested by previous authors. We discuss the origin of this structure and the results from the spectral analysis of HD826 under the scenario of a late thermal pulse.}, language = {en} } @article{ToalaOskinovaIgnace2017, author = {Toala, Jes{\´u}s Alberto and Oskinova, Lida and Ignace, R.}, title = {On the Absence of Non-thermal X-Ray Emission around Runaway O Stars}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {838}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2041-8205}, doi = {10.3847/2041-8213/aa667c}, pages = {1 -- 32}, year = {2017}, abstract = {Theoretical models predict that the compressed interstellar medium around runaway O stars can produce highenergy non-thermal diffuse emission, in particular, non-thermal X-ray and gamma-ray emission. So far, detection of nonthermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six welldetermined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward. zeta ph and BD+ 43 degrees 3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.}, language = {en} } @article{ToalaOskinovaGonzalezGalanetal.2016, author = {Toala, Jes{\´u}s Alberto and Oskinova, Lida and Gonzalez-Galan, Ana and Guerrero, Mart{\´i}n A. and Ignace, R. and Pohl, Martin}, title = {X-RAY OBSERVATIONS OF BOW SHOCKS AROUND RUNAWAY O STARS. THE CASE OF zeta OPH AND BD+43 degrees 3654}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {821}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/0004-637X/821/2/79}, pages = {9}, year = {2016}, abstract = {Non-thermal radiation has been predicted within bow shocks around runaway stars by recent theoretical works. We present X-ray observations toward the runaway stars zeta Oph by Chandra and Suzaku and of BD+43 degrees 3654 by XMM-Newton to search for the presence of non-thermal X-ray emission. We found no evidence of non-thermal emission spatially coincident with the bow shocks; nonetheless, diffuse emission was detected in the vicinity of zeta Oph. After a careful analysis of its spectral characteristics, we conclude that this emission has a thermal nature with a plasma temperature of T approximate to 2 x 10(6) K. The cometary shape of this emission seems to be in line with recent predictions of radiation-hydrodynamic models of runaway stars. The case of BD+43 degrees 3654 is puzzling, as non-thermal emission has been reported in a previous work for this source.}, language = {en} } @article{ToalaGuerreroTodtetal.2015, author = {Toala, Jes{\´u}s Alberto and Guerrero, Mart{\´i}n A. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A. and Sch{\"o}nberner, Detlef and Oskinova, Lida and Marquez-Lugo, R. A. and Fang, X. and Ramos-Larios, Gerardo}, title = {The born-again Planetary nebula A78: an X-RAY twin of A30}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {799}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/799/1/67}, pages = {10}, year = {2015}, abstract = {We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively.}, language = {en} } @article{SurlanHamannKubatetal.2012, author = {Surlan, B. and Hamann, Wolf-Rainer and Kubat, Jirij and Oskinova, Lida and Feldmeier, Achim}, title = {Three-dimensional radiative transfer in clumped hot star winds I influence of clumping on the resonance line formation}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {541}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201118590}, pages = {11}, year = {2012}, abstract = {Context. The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping has been found to be very important. To incorporate clumping into a radiative transfer calculation, three-dimensional (3D) models are required. Various properties of the clumps may have a strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. Aims. We incorporate the 3D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. Methods. We develop a full 3D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, we use realistic 3D models that describe the dense as well as the tenuous wind components to model the formation of resonance lines in a clumped stellar wind. At the same time, we account for non-monotonic velocity fields. Results. The 3D density and velocity wind inhomogeneities show that there is a very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3D models show that the line opacity is lower for a larger clump separation and shallower velocity gradients within the clumps. Conclusions. Our model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3D nature must be taken into account.}, language = {en} } @article{SurlanHamannAretetal.2013, author = {Surlan, B. and Hamann, Wolf-Rainer and Aret, A. and Kubat, Jiř{\´i} and Oskinova, Lida and Torres, A. F.}, title = {Macroclumping as solution of the discrepancy between Ha and P v mass loss diagnostics for O-type stars}, series = {ASTRONOMY \& ASTROPHYSICS}, volume = {559}, journal = {ASTRONOMY \& ASTROPHYSICS}, publisher = {EDP SCIENCES S A}, address = {LES ULIS CEDEX A}, issn = {0004-6361}, doi = {10.1051/0004-6361/201322390}, pages = {17}, year = {2013}, abstract = {Context. Recent studies of O-type stars have demonstrated that discrepant mass-loss rates are obtained when different diagnostic methods are employed. Fitting the unsaturated UV resonance lines (e.g., P v) gives drastically lower values than obtained from the Ha emission. Wind inhomogeneity (so-called "clumping") may be the main cause of this discrepancy. Aims. In a previous paper, we presented 3D Monte-Carlo calculations for the formation of scattering lines in a clumped stellar wind. In the present paper we select five O-type supergiants (from 04 to 07) and test whether the reported discrepancies can be resolved this way. Methods. In the first step, the analyses started with simulating the observed spectra with Potsdam Wolf-Rayet (PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit to the observed Ha emission lines best. For the unsaturated UV resonance lines (i.e., P v) we then applied our 3D Monte-Carlo code, which can account for wind clumps of any optical depths ("macroclumping"), a non-void interclump medium, and a velocity dispersion inside the clumps. The ionization stratifications and underlying photospheric spectra were adopted from the PoWR models. The properties of the wind clumps were constrained by fitting the observed resonance line profiles. Results. Our results show that with the mass-loss rates that fit Ha (and other Balmer and He II lines), the UV resonance lines (especially the unsaturated doublet of P v) can also be reproduced with no problem when macroclumping is taken into account. There is no need to artificially reduce the mass-loss rates or to assume a subsolar phosphorus abundance or an extremely high clumping factor, unlike what was claimed by other authors. These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared to the mass-loss rate recipe from Vink et al. Conclusions. Macroclumping resolves the previously reported discrepancy between Ha and P v mass-loss diagnostics.}, language = {en} } @article{SanjurjoFerrrinTorrejonPostnovetal.2017, author = {Sanjurjo-Ferrrin, G. and Torrejon, J. M. and Postnov, K. and Oskinova, Lida and Rodes-Roca, J. J. and Bernabeu, Guillermo}, title = {XMM-Newton spectroscopy of the accreting magnetar candidate 4U0114+65}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {606}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201630119}, pages = {4039 -- 4042}, year = {2017}, abstract = {Methods. We analysed the energy-resolved light curve and the time-resolved X-ray spectra provided by the EPIC cameras on board XMM-Newton. We also analysed the first high-resolution spectrum of this source provided by the Reflection Grating Spectrometer. Results. An X-ray pulse of 9350 +/- 160 s was measured. Comparison with previous measurements confirms the secular spin up of this source. We successfully fit the pulse-phase-resolved spectra with Comptonisation models. These models imply a very small (r similar to 3 km) and hot (kT similar to 2-3 keV) emitting region and therefore point to a hot spot over the neutron star (NS) surface as the most reliable explanation for the X-ray pulse. The long NS spin period, the spin-up rate, and persistent X-ray emission can be explained within the theory of quasi-spherical settling accretion, which may indicate that the magnetic field is in the magnetar range. Thus, 4U0114+65 could be a wind-accreting magnetar. We also observed two episodes of low luminosity. The first was only observed in the low-energy light curve and can be explained as an absorption by a large over-dense structure in the wind of the B1 supergiant donor. The second episode, which was deeper and affected all energies, may be due to temporal cessation of accretion onto one magnetic pole caused by non-spherical matter capture from the structured stellar wind. The light curve displays two types of dips that are clearly seen during the high-flux intervals. The short dips, with durations of tens of seconds, are produced through absorption by wind clumps. The long dips, in turn, seem to be associated with the rarefied interclump medium. From the analysis of the X-ray spectra, we found evidence of emission lines in the X-ray photoionised wind of the B1Ia donor. The Fe K alpha line was found to be highly variable and much weaker than in other X-ray binaries with supergiant donors. The degree of wind clumping, measured through the covering fraction, was found to be much lower than in supergiant donor stars with earlier spectral types. Conclusions. The XMM-Newton spectroscopy provided further support for the magnetar nature of the neutron star in 4U0114+65. The light curve presents dips that can be associated with clumps and the interclump medium in the stellar wind of the mass donor.}, language = {en} } @article{SanderVinkHamann2019, author = {Sander, Andreas Alexander Christoph and Vink, Jorick S. and Hamann, Wolf-Rainer}, title = {Driving classical Wolf-Rayet winds}, series = {Monthly notices of the Royal Astronomical Society}, volume = {491}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stz3064}, pages = {4406 -- 4425}, year = {2019}, abstract = {Classical Wolf-Rayet (cWR) stars are at a crucial evolutionary stage for constraining the fates of massive stars. The feedback of these hot, hydrogen-depleted stars dominates their surrounding by tremendous injections of ionizing radiation and kinetic energy. The strength of a Wolf-Rayet (WR) wind decides the eventual mass of its remnant, likely a massive black hole. However, despite their major influence and importance for gravitational wave detection statistics, WR winds are particularly poorly understood. In this paper, we introduce the first set of hydrodynamically consistent stellar atmosphere models for cWR stars of both the carbon (C) and the nitrogen (N) sequence, i.e. WC and WN stars, as a function of stellar luminosity-to-mass ratio (or Eddington Gamma) and metallicity. We demonstrate the inapplicability of the CAK wind theory for cWR stars and confirm earlier findings that their winds are launched at the (hot) iron (Fe) opacity peak. For log Z/Z(circle dot) > -2, Fe is also the main accelerator throughout the wind. Contrasting previous claims of a sharp lower mass-loss limit forWR stars, we obtain a smooth transition to optically thin winds. Furthermore, we find a strong dependence of the mass-loss rates on Eddington Gamma, both at solar and subsolar metallicity. Increases inWCcarbon and oxygen abundances turn out to slightly reduce the predicted mass-loss rates. Calculations at subsolar metallicities indicate that below the metallicity of the Small Magellanic Cloud, WR mass-loss rates decrease much faster than previously assumed, potentially allowing for high black hole masses even in the local Universe.}, language = {en} } @article{SanderShenarHainichetal.2015, author = {Sander, Andreas Alexander Christoph and Shenar, Tomer and Hainich, Rainer and Gimenez-Garcia, Ana and Todt, Helge Tobias and Hamann, Wolf-Rainer}, title = {On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {577}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201425356}, pages = {13}, year = {2015}, abstract = {Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres. Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra. Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses. Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50\% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates.}, language = {en} } @article{SanderHamannTodtetal.2017, author = {Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer and Todt, Helge Tobias and Hainich, Rainer and Shenar, Tomer}, title = {Coupling hydrodynamics with comoving frame radiative transfer I. A unified approach for OB and WR stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {603}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201730642}, pages = {14}, year = {2017}, abstract = {Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (nonLTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard beta-law occur.}, language = {en} } @article{SanderFuerstKretschmaretal.2018, author = {Sander, Andreas Alexander Christoph and F{\"u}rst, F. and Kretschmar, P. and Oskinova, Lida and Todt, Helge Tobias and Hainich, Rainer and Shenar, Tomer and Hamann, Wolf-Rainer}, title = {Coupling hydrodynamics with comoving frame radiative transfer}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {610}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731575}, pages = {19}, year = {2018}, abstract = {Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.}, language = {en} } @article{SanderHamannTodt2012, author = {Sander, A. and Hamann, Wolf-Rainer and Todt, Helge Tobias}, title = {The Galactic WC stars Stellar parameters from spectral analyses indicate a new evolutionary sequence}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {540}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117830}, pages = {79}, year = {2012}, abstract = {Context. The life cycles of massive stars from the main sequence to their explosion as supernovae or gamma ray bursts are not yet fully clear, and the empirical results from spectral analyses are partly in conflict with current evolutionary models. The spectral analysis of Wolf-Rayet stars requires the detailed modeling of expanding stellar atmospheres in non-LTE. The Galactic WN stars have been comprehensively analyzed with such models of the latest stage of sophistication, while a similarly comprehensive study of the Galactic WC sample remains undone. Aims. We aim to establish the stellar parameters and mass-loss rates of the Galactic WC stars. These data provide the empirical basis of studies of (i) the role of WC stars in the evolution of massive stars, (ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as input to models of the chemical and dynamical evolution of galaxies. Methods. We analyze the nearly complete sample of un-obscured Galactic WC stars, using optical spectra as well as ultraviolet spectra when available. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. A large grid of line-blanked models has been established for the range of WC subtypes WC4 - WC8, and smaller grids for the WC9 parameter domain. Both WO stars and WN/WC transit types are also analyzed using special models. Results. Stellar and atmospheric parameters are derived for more than 50 Galactic WC and two WO stars, covering almost the whole Galactic WC population as far as the stars are single, and un-obscured in the visual. In the Hertzsprung-Russell diagram, the WC stars reside between the hydrogen and the helium zero-age main sequences, having luminosities L from 10(4.9) to 10(5.6) L-circle dot. The mass-loss rates scale very tightly with L-0.8. The two WO stars in our sample turn out to be outstandingly hot (approximate to 200 kK) and do not fit into the WC scheme. Conclusions. By comparing the empirical WC positions in the Hertzsprung-Russell diagram with evolutionary models, and from recent supernova statistics, we conclude that WC stars have evolved from initial masses between 20 solar masses and 45 M-circle dot. In contrast to previous assumptions, it seems that WC stars in general do not descend from the most massive stars. Only the WO stars might stem from progenitors that have been initially more massive than 45 M-circle dot.}, language = {en} } @article{SanchezAyasodelValleMartietal.2018, author = {Sanchez-Ayaso, Mar{\´i}a de la Estrella and del Valle, Maria Victoria and Marti, Josep and Romero, G. E. and Luque-Escamilla, Pedro Luis}, title = {Possible association of two Stellar Bowshocks with Unidentified Fermi Sources}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {861}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aac7c7}, pages = {9}, year = {2018}, abstract = {The bowshocks of runaway stars had been theoretically proposed as gamma-ray sources. However, this hypothesis has not been confirmed by observations to date. In this paper, we present two runaway stars (lambda Cep and LS 2355) whose bowshocks are coincident with the unidentified Fermi gamma-ray sources 3FLG J2210.1+5925 and 3FGL J1128.7-6232, respectively. After performing a cross-correlation between different catalogs at distinct wavelengths, we found that these bowshocks are the most peculiar objects in the Fermi position ellipses. Then we computed the inverse Compton emission and fitted the Fermi data in order to test the viability of both runaway stars as potential counterparts of the two high-energy sources. We obtained very reasonable values for the fitted parameters of both stars. We also evaluated the possibility for the source 3FGL J1128.7-6232, which is positionally coincident with a H II region, to be the result of background cosmic-ray protons interacting with the matter of the cloud, as well as the probability of a pure chance association. We conclude that the gamma rays from these Fermi sources might be produced in the bowshocks of the considered runaway stars. In such a case, these would be the first sources of this class ever detected at gamma rays.}, language = {en} } @article{RichardsonShenarRoyLoubieretal.2016, author = {Richardson, Noel D. and Shenar, Tomer and Roy-Loubier, Olivier and Schaefer, Gail and Moffat, Anthony F. J. and St-Louis, Nicole and Gies, Douglas R. and Farrington, Chris and Hill, Grant M. and Williams, Peredur M. and Gordon, Kathryn and Pablo, Herbert and Ramiaramanantsoa, Tahina}, title = {The CHARA Array resolves the long-period Wolf-Rayet binaries WR 137 and WR 138}, series = {Monthly notices of the Royal Astronomical Society}, volume = {461}, journal = {Monthly notices of the Royal Astronomical Society}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stw1585}, pages = {4115 -- 4124}, year = {2016}, abstract = {We report on interferometric observations with the CHARAArray of two classical Wolf-Rayet (WR) stars in suspected binary systems, namely WR 137 and WR 138. In both cases, we resolve the component stars to be separated by a few milliarcseconds. The data were collected in the H band, and provide a measure of the fractional flux for both stars in each system. We find that the WR star is the dominant H-band light source in both systems (fWR, 137 = 0.59 +/- 0.04; fWR, 138 = 0.67 +/- 0.01), which is confirmed through both comparisons with estimated fundamental parameters for WR stars and O dwarfs, as well as through spectral modelling of each system. Our spectral modelling also provides fundamental parameters for the stars and winds in these systems. The results on WR 138 provide evidence that it is a binary system which may have gone through a previous mass-transfer episode to create the WR star. The separation and position of the stars in the WR 137 system together with previous results from the IOTA interferometer provides evidence that the binary is seen nearly edgeon. The possible edge-on orbit of WR 137 aligns well with the dust production site imaged by the Hubble Space Telescope during a previous periastron passage, showing that the dust production may be concentrated in the orbital plane.}, language = {en} } @article{RamosLariosToalaRodriguezGonzalezetal.2022, author = {Ramos-Larios, Gerardo and Toala, Jes{\´u}s Alberto and Rodriguez-Gonzalez, Janis B. and Guerrero, Martin A. and Gomez-Gonzalez, V{\´i}ctor Mauricio Alfonso}, title = {Rings and arcs around evolved stars - III. Physical conditions of the ring-like structures in the planetary nebula IC 4406 revealed by MUSE}, series = {Monthly notices of the Royal Astronomical Society}, volume = {513}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, pages = {2862 -- 2868}, year = {2022}, abstract = {We present the analysis of Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of the planetary nebula (PN) IC 4406. MUSE images in key emission lines are used to unveil the presence of at least five ring-like structures north and south of the main nebula of IC4406. MUSE spectra are extracted from the rings to unambiguously assess for the first time in a PN their physical conditions, electron density (n(e)), and temperature (T-e). The rings are found to have similar T-e as the rim of the main nebula, but smaller n(e). Ratios between different ionic species suggest that the rings of IC4406 have a lower ionization state than the main cavity, in contrast to what was suggested for the rings in NGC 6543, the Cat's Eye Nebula.}, language = {en} } @article{RamiaramanantsoaMoffatHarmonetal.2018, author = {Ramiaramanantsoa, Tahina and Moffat, Anthony F. J. and Harmon, Robert and Ignace, R. and St-Louis, Nicole and Vanbeveren, Dany and Shenar, Tomer and Pablo, Herbert and Richardson, Noel D. and Howarth, Ian D. and Stevens, Ian R. and Piaulet, Caroline and St-Jean, Lucas and Eversberg, Thomas and Pigulski, Andrzej and Popowicz, Adam and Kuschnig, Rainer and Zoclonska, Elzbieta and Buysschaert, Bram and Handler, Gerald and Weiss, Werner W. and Wade, Gregg A. and Rucinski, Slavek M. and Zwintz, Konstanze and Luckas, Paul and Heathcote, Bernard and Cacella, Paulo and Powles, Jonathan and Locke, Malcolm and Bohlsen, Terry and Chen{\´e}, Andr{\´e}-Nicolas and Miszalski, Brent and Waldron, Wayne L. and Kotze, Marissa M. and Kotze, Enrico J. and B{\"o}hm, Torsten}, title = {BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant zeta Puppis unveils the photospheric drivers of its small- and large-scale wind structures}, series = {Monthly notices of the Royal Astronomical Society}, volume = {473}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stx2671}, pages = {5532 -- 5569}, year = {2018}, abstract = {From 5.5 months of dual-band optical photometric monitoring at the 1 mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star ζ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the 1.78-d signal previously detected by Coriolis/Solar Mass Ejection Imager, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inhomogeneities. By means of a constrained non-linear light-curve inversion algorithm, we mapped the locations of the bright surface spots and traced their evolution. Our simultaneous ground-based multisite spectroscopic monitoring of the star unveiled cyclical modulation of its He ii λ4686 wind emission line with the 1.78-d rotation period, showing signatures of corotating interaction regions that turn out to be driven by the bright photospheric spots observed by BRITE. Traces of wind clumps are also observed in the He ii λ4686 line and are correlated with the amplitudes of the stochastic component of the light variations probed by BRITE at the photosphere, suggesting that the BRITE observations additionally unveiled the photospheric drivers of wind clumps in ζ Pup and that the clumping phenomenon starts at the very base of the wind. The origins of both the bright surface inhomogeneities and the stochastic light variations remain unknown, but a subsurface convective zone might play an important role in the generation of these two types of photospheric variability.}, language = {en} } @article{RamachandranHamannHainichetal.2018, author = {Ramachandran, Varsha and Hamann, Wolf-Rainer and Hainich, Rainer and Oskinova, Lida and Shenar, Tomer and Sander, Andreas Alexander Christoph and Todt, Helge Tobias and Gallagher, John S.}, title = {Stellar population of the superbubble N206 in the LMC II. Parameters of the OB and WR stars, and the total massive star feedback}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {615}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201832816}, pages = {72}, year = {2018}, abstract = {Context. Clusters or associations of early-type stars are often associated with a "superbubble" of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud (LMC) exhibits a superbubble and a rich massive star population. Aims. Our goal is to perform quantitative spectral analyses of all massive stars associated with the N206 superbubble in order to determine their stellar and wind parameters. We compare the superbubble energy budget to the stellar energy input and discuss the star formation history of the region. Results. We present the stellar and wind parameters of the OB stars and the two Wolf-Rayet (WR) binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The Hertzsprung-Russell diagram (HRD) of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. The total ionizing photon flux produced by all massive stars in the N206 complex is Q(0) approximate to 5 x 10(50) s(-1), and the mechanical luminosity of their stellar winds amounts to L-mec = 1.7 x 10(38) erg s(-1). Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is approximate to 2.3 x 10(52) erg. Conclusions. The N206 complex in the LMC has undergone star formation episodes since more than 30 Myr ago. From the spectral analyses of its massive star population, we derive a current star formation rate of 2.2 x 10(-3) M-circle dot yr(-1). From the combined input of mechanical energy from all stellar winds, only a minor fraction is emitted in the form of X-rays. The corresponding input accumulated over a long time also exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.}, language = {en} } @article{RamachandranHainichHamannetal.2017, author = {Ramachandran, Varsha and Hainich, Rainer and Hamann, Wolf-Rainer and Oskinova, Lida and Shenar, T. and Sander, Andreas Alexander Christoph and Todt, Helge Tobias and Gallagher, John S.}, title = {Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {609}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731093}, pages = {26}, year = {2017}, abstract = {Context. Massive stars severely influence their environment by their strong ionizing radiation and by the momentum and kinetic energy input provided by their stellar winds and supernovae. Quantitative analyses of massive stars are required to understand how their feedback creates and shapes large scale structures of the interstellar medium. The giant H II region N206 in the Large Magellanic Cloud contains an OB association that powers a superbubble filled with hot X-ray emitting gas, serving as an ideal laboratory in this context. Aims. We aim to estimate stellar and wind parameters of all OB stars in N206 by means of quantitative spectroscopic analyses. In this first paper, we focus on the nine Of-type stars located in this region. We determine their ionizing flux and wind mechanical energy. The analysis of nitrogen abundances in our sample probes rotational mixing. Methods. We obtained optical spectra with the multi-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we used the Potsdam Wolf-Rayet model atmosphere code. We determined the physical parameters and nitrogen abundances of our sample stars by fitting synthetic spectra to the observations. Results. The stellar and wind parameters of nine Of-type stars, which are largely derived from spectral analysis are used to construct wind momentum luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant that has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age of less than 4 million yr, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. Conclusions. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.}, language = {en} } @article{ParkinBroosTownsleyetal.2011, author = {Parkin, E. R. and Broos, Patrick S. and Townsley, L. K. and Pittard, J. M. and Moffat, Anthony F. J. and Naze, Y. and Rauw, G. and Oskinova, Lida and Waldron, W. L.}, title = {X-RAY EMISSION FROM THE DOUBLE-BINARY OB-STAR SYSTEM QZ CAR (HD 93206)}, series = {ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, volume = {194}, journal = {ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, number = {1}, publisher = {IOP PUBLISHING LTD}, address = {BRISTOL}, issn = {0067-0049}, doi = {10.1088/0067-0049/194/1/8}, pages = {11}, year = {2011}, abstract = {X-ray observations of the double-binary OB-star system QZ Car (HD 93206) obtained with the Chandra X-ray Observatory over a period of roughly 2 years are presented. The respective orbits of systems A (O9.7 I+b2 v, P-A = 21 days) and B (O8 III+o9 v, P-B = 6 days) are reasonably well sampled by the observations, allowing the origin of the X-ray emission to be examined in detail. The X-ray spectra can be well fitted by an attenuated three-temperature thermal plasma model, characterized by cool, moderate, and hot plasma components at kT similar or equal to 0.2, 0.7, and 2 keV, respectively, and a circumstellar absorption of similar or equal to 0.2 x 10(22) cm(-2). Although the hot plasma component could be indicating the presence of wind-wind collision shocks in the system, the model fluxes calculated from spectral fits, with an average value of similar or equal to 7x10(-13) erg s(-1) cm(-2), do not show a clear correlation with the orbits of the two constituent binaries. A semi-analytical model of QZ Car reveals that a stable momentum balance may not be established in either system A or B. Yet, despite this, system B is expected to produce an observed X-ray flux well in excess of the observations. If one considers the wind of the O8 III star to be disrupted by mass transfer, the model and observations are in far better agreement, which lends support to the previous suggestion of mass transfer in the O8 III+o9 v binary. We conclude that the X-ray emission from QZ Car can be reasonably well accounted for by a combination of contributions mainly from the single stars and the mutual wind-wind collision between systems A and B.}, language = {en} } @article{OskinovaSunEvansetal.2013, author = {Oskinova, Lida and Sun, W. and Evans, C. J. and Henault-Brunet, V. and Chu, Y.-H. and Gallagher, J. S. and Guerrero, Mart{\´i}n A. and Gruendl, R. A. and G{\"u}del, M. and Silich, S. and Chen, Y. and Naze, Y. and Hainich, Rainer and Reyes-Iturbide, J.}, title = {Discovery of x-ray emission from young suns in the small magellanic cloud}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {765}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/765/1/73}, pages = {12}, year = {2013}, abstract = {We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-Ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H II region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low-and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the newborn stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low-and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.}, language = {en} } @article{OskinovaHuenemoerderHamannetal.2017, author = {Oskinova, Lida and Huenemoerder, D. P. and Hamann, Wolf-Rainer and Shenar, Tomer and Sander, Andreas Alexander Christoph and Ignace, R. and Todt, Helge Tobias and Hainich, Rainer}, title = {On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {845}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aa7e79}, pages = {11}, year = {2017}, abstract = {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.}, language = {en} } @article{OskinovaHamannCassinellietal.2011, author = {Oskinova, Lida and Hamann, Wolf-Rainer and Cassinelli, Joseph P. and Brown, John C. and Todt, Helge Tobias}, title = {X-ray emission from massive stars with magnetic fields}, series = {Astronomische Nachrichten = Astronomical notes}, volume = {332}, journal = {Astronomische Nachrichten = Astronomical notes}, number = {9-10}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0004-6337}, doi = {10.1002/asna.201111602}, pages = {988 -- 993}, year = {2011}, abstract = {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.}, language = {en} } @article{OskinovaGayleyHamannetal.2012, author = {Oskinova, Lida and Gayley, K. G. and Hamann, Wolf-Rainer and Huenemoerder, D. P. and Ignace, R. and Pollock, A. M. T.}, title = {HIGH-RESOLUTION X-RAY SPECTROSCOPY REVEALS THE SPECIAL NATURE OF WOLF-RAYET STAR WINDS}, series = {ASTROPHYSICAL JOURNAL LETTERS}, volume = {747}, journal = {ASTROPHYSICAL JOURNAL LETTERS}, number = {2}, publisher = {IOP PUBLISHING LTD}, address = {BRISTOL}, issn = {2041-8205}, doi = {10.1088/2041-8205/747/2/L25}, pages = {6}, year = {2012}, abstract = {We present the first high-resolutionX-ray spectrum of a putatively singleWolf-Rayet (WR) star. 400 ks observations of WR 6 by the XMM-Newton telescope resulted in a superb quality high-resolution X-ray spectrum. Spectral analysis reveals that the X-rays originate far out in the stellar wind, more than 30 stellar radii from the photosphere, and thus outside the wind acceleration zone where the line-driving instability (LDI) could create shocks. The X-ray emitting plasma reaches temperatures up to 50 MK and is embedded within the unshocked, "cool" stellar wind as revealed by characteristic spectral signatures. We detect a fluorescent Fe line at approximate to 6.4 keV. The presence of fluorescence is consistent with a two-component medium, where the cool wind is permeated with the hot X-ray emitting plasma. The wind must have a very porous structure to allow the observed amount of X-rays to escape. We find that neither the LDI nor any alternative binary scenario can explain the data. We suggest a scenario where X-rays are produced when the fast wind rams into slow "sticky clumps" that resist acceleration. Our new data show that the X-rays in single WR star are generated by some special mechanism different from the one operating in the O-star winds.}, language = {en} } @article{MunozMoffatHilletal.2017, author = {Munoz, Melissa and Moffat, Anthony F. J. and Hill, Grant M. and Shenar, Tomer and Richardson, Noel D. and Pablo, Herbert and St-Louis, Nicole and Ramiaramanantsoa, Tahina}, title = {WR 148: identifying the companion of an extreme runaway massive binary}, series = {Monthly notices of the Royal Astronomical Society}, volume = {467}, journal = {Monthly notices of the Royal Astronomical Society}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stw2283}, pages = {3105 -- 3121}, year = {2017}, abstract = {WR 148 (HD 197406) is an extreme runaway system considered to be a potential candidate for a short-period (4.3173 d) rare WR + compact object binary. Provided with new high-resolution, high signal-to-noise spectra from the Keck observatory, we determine the orbital parameters for both the primary WR and the secondary, yielding respective projected orbital velocity amplitudes of 88.1 ± 3.8\&\#8201;km\&\#8201;s\&\#8722;1 and 79.2 ± 3.1\&\#8201;km\&\#8201;s\&\#8722;1 and implying a mass ratio of 1.1 ± 0.1. We then apply the shift-and-add technique to disentangle the spectra and obtain spectra compatible with a WN7ha and an O4-6 star. Considering an orbital inclination of \&\#8764;67°, derived from previous polarimetry observations, the system's total mass would be a mere 2-3M\&\#8857;\&\#8288;, an unprecedented result for a putative massive binary system. However, a system comprising a 37M\&\#8857; secondary (typical mass of an O5V star) and a 33M\&\#8857; primary (given the mass ratio) would infer an inclination of \&\#8764;18°. We therefore reconsider the previous methods of deriving the orbital inclination based on time-dependent polarimetry and photometry. While the polarimetric results are inconclusive requiring better data, the photometric results favour low inclinations. Finally, we compute WR 148's space velocity and retrace the runaway's trajectory back to the Galactic plane (GP). With an ejection velocity of 198 ± 27\&\#8201;km\&\#8201;s\&\#8722;1 and a travel time of 4.7 ± 0.8 Myr to reach its current location, WR 148 was most likely ejected via dynamical interactions in a young cluster.}, language = {en} } @article{MassaOskinovaPrinjaetal.2019, author = {Massa, Derck and Oskinova, Lida and Prinja, Raman and Ignace, Richard}, title = {Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant xi Per}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {873}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/ab0283}, pages = {12}, year = {2019}, abstract = {We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7. III(n) ((f)) star xi Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of xi Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N IV lambda 1718 and Si IV lambda 1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si IV lambda 1402, N IV lambda 1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180 degrees, on the surface of the star. We note that the presence and persistence of two spots separated by 180 degrees suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in xi Per.}, language = {en} } @article{MassaOskinovaFullertonetal.2014, author = {Massa, D. and Oskinova, Lida and Fullerton, A. W. and Prinja, R. K. and Bohlender, D. A. and Morrison, N. D. and Blake, M. and Pych, W.}, title = {CIR modulation of the X-ray flux from the O7.5 III(n)((f)) star xi Persei(a similar to...)?}, series = {Monthly notices of the Royal Astronomical Society}, volume = {441}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu565}, pages = {2173 -- 2180}, year = {2014}, abstract = {We analyse a 162 ks high energy transmission grating Chandra observation of the O7.5 III(n)((f)) star xi Per, together with contemporaneous H alpha observations. The X-ray spectrum of this star is similar to other single O stars, and not pathological in any way. Its UV wind lines are known to display cyclical time variability, with a period of 2.086 d, which is thought to be associated with corotating interaction regions (CIRs). We examine the Chandra and H alpha data for variability on this time-scale. We find that the X-rays vary by similar to 15 per cent over the course of the observations and that this variability is out of phase with variable absorption on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV absorption associated with CIRs). While not conclusive, both sets of data are consistent with models where the CIRs are either a source of X-rays or modulate them.}, language = {en} } @article{KustererNagelHartmannetal.2014, author = {Kusterer, D. -J. and Nagel, T. and Hartmann, S. and Werner, K. and Feldmeier, Achim}, title = {Monte Carlo radiation transfer in CV disk winds: application to the AM CVn prototype}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {561}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201321438}, pages = {10}, year = {2014}, abstract = {Context. AMCVn systems are ultracompact binaries in which a (semi-) degenerate star transfers helium-dominated matter onto a white dwarf. They are effective gravitational-wave emitters and potential progenitors of Type Ia supernovae. Aims. To understand the evolution of AMCVn systems it is necessary to determine their mass-loss rate through their radiation-driven accretion-disk wind. We constructed models to perform quantitative spectroscopy of P Cygni line profiles that were detected in UV spectra. Methods. We performed 2.5D Monte Carlo radiative transfer calculations in hydrodynamic wind structures by making use of realistic NLTE spectra from the accretion disk and by accounting for the white dwarf as an additional photon source. Results. We present first results from calculations in which LTE opacities are used in the wind model. A comparison with UV spectroscopy of the AMCVn prototype shows that the modeling procedure is potentially a good tool for determining mass-loss rates and abundances of trace metals in the helium-rich wind.}, language = {en} } @article{KurfuerstFeldmeierKrticka2018, author = {Kurf{\"u}rst, P. and Feldmeier, Achim and Krticka, Jiri}, title = {Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {613}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731300}, pages = {24}, year = {2018}, abstract = {Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. It is also unclear how various forming physical mechanisms of the circumstellar environment affect its shape and density, as well as its kinematic and thermal structure. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk for mass-loss rates higher than (M) over dot greater than or similar to 10(-10) M-circle dot yr(-1). In the models of dense viscous disks with (M) over dot > 10(-8) M-circle dot yr(-1), the viscosity increases the central temperature up to several tens of thousands of Kelvins, however the temperature rapidly drops with radius and with distance from the disk midplane. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen, for example.}, language = {en} } @article{KubatovaSzecsiSanderetal.2019, author = {Kubatova, Brankica and Szecsi, D. and Sander, Andreas Alexander Christoph and Kubat, Jiř{\´i} and Tramper, F. and Krticka, Jiri and Kehrig, C. and Hamann, Wolf-Rainer and Hainich, Rainer and Shenar, Tomer}, title = {Low-metallicity massive single stars with rotation}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {623}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201834360}, pages = {32}, year = {2019}, abstract = {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.}, language = {en} } @article{KrtickaFeldmeier2018, author = {Krticka, Jiri and Feldmeier, Achim}, title = {Light variations due to the line-driven wind instability and wind blanketing in O stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {617}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731614}, pages = {7}, year = {2018}, abstract = {A small fraction of the radiative flux emitted by hot stars is absorbed by their winds and redistributed towards longer wavelengths. This effect, which leads also to the heating of the stellar photosphere, is termed wind blanketing. For stars with variable winds, the effect of wind blanketing may lead to the photometric variability. We have studied the consequences of line driven wind instability and wind blanketing for the light variability of O stars. We combined the results of wind hydrodynamic simulations and of global wind models to predict the light variability of hot stars due to the wind blanketing and instability. The wind instability causes stochastic light variability with amplitude of the order of tens of millimagnitudes and a typical timescale of the order of hours for spatially coherent wind structure. The amplitude is of the order of millimagnitudes when assuming that the wind consists of large number of independent concentric cones. The variability with such amplitude is observable using present space borne photometers. We show that the simulated light curve is similar to the light curves of O stars obtained using BRITE and CoRoT satellites.}, language = {en} } @article{IgnaceGayleyHamannetal.2013, author = {Ignace, Rico and Gayley, Kenneth G. and Hamann, Wolf-Rainer and Huenemoerder, David P. and Oskinova, Lida and Pollock, Andy M. T. and McFall, Michael}, title = {THE XMM-NEWTON/EPIC X-RAY LIGHT CURVE ANALYSIS OF WR 6}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {775}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/775/1/29}, pages = {12}, year = {2013}, abstract = {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.}, language = {en} } @article{HamannGraefenerLiermannetal.2019, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G. and Liermann, A. and Hainich, Rainer and Sander, Andreas Alexander Christoph and Shenar, Tomer and Ramachandran, Varsha and Todt, Helge Tobias and Oskinova, Lida}, title = {The Galactic WN stars revisited}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {625}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201834850}, pages = {11}, year = {2019}, abstract = {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.}, language = {en} } @article{HamaguchiOskinovaRusselletal.2016, author = {Hamaguchi, K. and Oskinova, Lida and Russell, C. M. P. and Petre, R. and Enoto, T. and Morihana, K. and Ishida, M.}, title = {DISCOVERY OF RAPIDLY MOVING PARTIAL X-RAY ABSORBERS WITHIN GAMMA CASSIOPEIAE}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {832}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/0004-637X/832/2/140}, pages = {33 -- 49}, year = {2016}, abstract = {detected six rapid X-ray spectral hardening events called "softness dips" in a similar to 100 ks observation in 2011. All the softness dip events show symmetric softness-ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either similar to 40\% or similar to 70\% partial covering absorption to kT similar to 12 keV plasma emission by matter with a neutral hydrogen column density of similar to(2-8) x 10(21) cm(-2), while the spectrum outside these dips is almost free of absorption. This result suggests the presence of two distinct X-ray-emitting spots in the.. Cas system, perhaps on a white dwarf (WD) companion with dipole mass accretion. The partial covering absorbers may be blobs in the Be stellar wind, the Be disk, or rotating around the WD companion. Weak correlations of the softness ratios to the hard X-ray flux suggest the presence of stable plasmas at kT similar to 0.9 and 5 keV, which may originate from the Be or WD winds. The formation of a Be star and WD binary system requires mass transfer between two stars; gamma Cas may have experienced such activity in the past.}, language = {en} } @article{HainichRuehlingTodtetal.2014, author = {Hainich, Rainer and Ruehling, Ute and Todt, Helge Tobias and Oskinova, Lida and Liermann, A. and Graefener, G. and Foellmi, C. and Schnurr, O. and Hamann, Wolf-Rainer}, title = {The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {565}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201322696}, pages = {62}, year = {2014}, abstract = {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.}, language = {en} } @article{HainichPasemannTodtetal.2015, author = {Hainich, Rainer and Pasemann, Diana and Todt, Helge Tobias and Shenar, Tomer and Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer}, title = {Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {581}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201526241}, pages = {30}, year = {2015}, abstract = {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.}, language = {en} } @article{GuerreroRuizHamannetal.2012, author = {Guerrero, Mart{\´i}n A. and Ruiz, N. and Hamann, Wolf-Rainer and Chu, Y.-H. and Todt, Helge Tobias and Sch{\"o}nberner, Detlef and Oskinova, Lida and Gr{\"u}ndl, R. A. and Steffen, M. and Blair, William P. and Toala, Jes{\´u}s Alberto}, title = {Rebirth of X-Ray emission from the born-again planetary Nebula A30}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {755}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/755/2/129}, pages = {15}, year = {2012}, abstract = {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.}, language = {en} } @article{GrinbergHellElMellahetal.2017, author = {Grinberg, Victoria and Hell, Natalie and El Mellah, Ileyk and Neilsen, Joseph and Sander, Andreas Alexander Christoph and Leutenegger, Maurice and F{\"u}rst, Felix and Huenemoerder, David P. and Kretschmar, Peter and Kuehnel, Matthias and Martinez-Nunez, Silvia and Niu, Shu and Pottschmidt, Katja and Schulz, Norbert S. and Wilms, Joern and Nowak, Michael A.}, title = {The clumpy absorber in the high-mass X-ray binary Vela X-1}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {608}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731843}, pages = {18}, year = {2017}, abstract = {Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase similar to 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium, and neon whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.}, language = {en} } @article{GimenezGarciaTorrejonEikmannetal.2015, author = {Gimenez-Garcia, Angel and Torrejon, Jose Miguel and Eikmann, Wiebke and Martinez-Nunez, Silvia and Oskinova, Lida and Rodes-Roca, Jose Joaquin and Bernabeu, Guillermo}, title = {An XMM-Newton view of FeK alpha in high-mass X-ray binaries}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {576}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201425004}, pages = {31}, year = {2015}, abstract = {We present a comprehensive analysis of the whole sample of available XMM-Newton observations of high-mass X-ray binaries (HMXBs) until August 2013, focusing on the FeK alpha emission line. This line is key to better understanding the physical properties of the material surrounding the X-ray source within a few stellar radii (the circumstellar medium). We collected observations from 46 HMXBs and detected FeK alpha in 21 of them. We used the standard classification of HMXBs to divide the sample into different groups. We find that (1) different classes of HMXBs display different qualitative behaviours in the FeK alpha spectral region. This is visible especially in SGXBs (showing ubiquitous Fe fluorescence but not recombination Fe lines) and in gamma Cass analogues (showing both fluorescent and recombination Fe lines). (2) FeK alpha is centred at a mean value of 6.42 keV. Considering the instrumental and fits uncertainties, this value is compatible with ionization states that are lower than Fe xviii. (3) The flux of the continuum is well correlated with the flux of the line, as expected. Eclipse observations show that the Fe fluorescence emission comes from an extended region surrounding the X-ray source. (4) We observe an inverse correlation between the X-ray luminosity and the equivalent width of FeK alpha (EW). This phenomenon is known as the X-ray Baldwin effect. (5) FeK alpha is narrow (sigma(line) < 0.15 keV), reflecting that the reprocessing material does not move at high speeds. We attempt to explain the broadness of the line in terms of three possible broadening phenomena: line blending, Compton scattering, and Doppler shifts (with velocities of the reprocessing material V similar to 1000 km s(-1)). (6) The equivalent hydrogen column (N-H) directly correlates to the EW of FeK alpha, displaying clear similarities to numerical simulations. It highlights the strong link between the absorbing and the fluorescent matter. (7) The observed NH in supergiant X-ray binaries (SGXBs) is in general higher than in supergiant fast X-ray transients (SFXTs). We suggest two possible explanations: different orbital configurations or a different interaction compact object - wind. (8) Finally, we analysed the sources IGR J16320-4751 and 4U 1700-37 in more detail, covering several orbital phases. The observed variation in NH between phases is compatible with the absorption produced by the wind of their optical companions. The results clearly point to a very important contribution of the donor's wind in the FeK alpha emission and the absorption when the donor is a supergiant massive star.}, language = {en} } @article{GimenezGarciaShenarTorrejonetal.2016, author = {Gimenez-Garcia, Ana and Shenar, Tomer and Torrejon, J. M. and Oskinova, Lida and Martinez-Nunez, S. and Hamann, Wolf-Rainer and Rodes-Roca, J. J. and Gonz{\´a}lez-Galan, A. and Alonso-Santiago, J. and Gonz{\´a}lez-Fern{\´a}ndez, C. and Bernabeu, Guillermo and Sander, Andreas Alexander Christoph}, title = {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}, series = {Siberian Mathematical Journal}, volume = {591}, journal = {Siberian Mathematical Journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201527551}, pages = {25}, year = {2016}, abstract = {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.}, language = {en} } @article{ElMellahSanderSundqvistetal.2019, author = {El Mellah, Ileyk and Sander, Andreas Alexander Christoph and Sundqvist, Jon Olof and Keppens, Rony}, title = {Formation of wind-captured disks in supergiant X-ray binaries Consequences for Vela X-1 and Cygnus X-1}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {622}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201834498}, pages = {12}, year = {2019}, abstract = {Context. In supergiant X-ray binaries (SgXB), a compact object captures a fraction of the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the efficiency of mass and angular momentum accretion, but they depend so dramatically on the wind speed that given the current uncertainties, they only set loose constraints. Furthermore, these proxies often bypass the impact of orbital and shock effects on the flow structure. Aims. We study the wind dynamics and angular momentum gained as the flow is accreted. We identify the conditions for the formation of a disk-like structure around the accretor and the observational consequences for SgXB. Methods. We used recent results on the wind launching mechanism to compute 3D streamlines, accounting for the gravitational and X-ray ionizing influence of the compact companion on the wind. Once the flow enters the Roche lobe of the accretor, we solved the hydrodynamics equations with cooling. Results. A shocked region forms around the accretor as the flow is beamed. For wind speeds on the order of the orbital speed, the shock is highly asymmetric compared to the axisymmetric bow shock obtained for a purely planar homogeneous flow. With net radiative cooling, the flow always circularizes for sufficiently low wind speeds. Conclusions. Although the donor star does not fill its Roche lobe, the wind can be significantly beamed and bent by the orbital effects. The net angular momentum of the accreted flow is then sufficient to form a persistent disk-like structure. This mechanism could explain the proposed limited outer extension of the accretion disk in Cygnus X-1 and suggests the presence of a disk at the outer rim of the neutron star magnetosphere in Vela X-1 and has dramatic consequences on the spinning up of the accretor.}, language = {en} } @article{delVallePohl2018, author = {del Valle, Maria Victoria and Pohl, Martin}, title = {Nonthermal emission from Stellar Bow Shocks}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {864}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aad333}, pages = {14}, year = {2018}, abstract = {Since the detection of nonthermal radio emission from the bow shock of the massive runaway star BD +43 degrees 3654, simple models have predicted high-energy emission, at X-rays and gamma-rays, from these Galactic sources. Observational searches for this emission so far give no conclusive evidence but a few candidates at gamma-rays. In this work we aim at developing a more sophisticated model for the nonthermal emission from massive runaway star bow shocks. The main goal is to establish whether these systems are efficient nonthermal emitters, even if they are not strong enough yet to be detected. For modeling the collision between the stellar wind and the interstellar medium we use 2D hydrodynamic simulations. We then adopt the flow profile of the wind and the ambient medium obtained with the simulation as the plasma state for solving the transport of energetic particles injected in the system, as well as the nonthermal emission they produce. For this purpose we solve a 3D (two spatial vertical bar energy) advection-diffusion equation in the test-particle approximation. We find that a massive runaway star with a powerful wind converts 0.16\%-0.4\% of the power injected in electrons into nonthermal emission, mostly produced by inverse Compton scattering of dust-emitted photons by relativistic electrons, and second by synchrotron radiation. This represents a fraction of similar to 10(-5) to 10(-4) of the wind kinetic power. Given the better sensibility of current instruments at radio wavelengths, these systems are more prone to be detected at radio through the synchrotron emission they produce rather than at gamma energies.}, language = {en} } @article{AbdallaAharonianBenkhalietal.2019, author = {Abdalla, Hassan E. and Aharonian, Felix A. and Benkhali, F. Ait and Ang{\"u}ner, Ekrem Oǧuzhan and Arakawa, M. and Arcaro, C. and Armand, C. and Backes, M. and Barnard, M. and Becherini, Y. and Berge, D. and Bernloehr, K. and Blackwell, R. and Bottcher, M. and Boisson, C. and Bolmont, J. and Bonnefoy, S. and Bregeon, J. and Brun, F. and Brun, P. and Bryan, M. and Buechele, M. and Bulik, T. and Bylund, T. and Capasso, M. and Caroff, S. and Carosi, A. and Casanova, Sabrina and Cerruti, M. and Chakraborty, N. and Chand, T. and Chandra, S. and Chaves, R. C. G. and Chen, A. and Colafrancesco, S. and Condon, B. and Davids, I. D. and Deil, C. and Devin, J. and deWilt, P. and Dirson, L. and Djannati-Atai, A. and Dmytriiev, A. and Donath, A. and Doroshenko, V and Dyks, J. and Egberts, Kathrin and Emery, G. and Ernenwein, J-P and Eschbach, S. and Feijen, K. and Fegan, S. and Fiasson, A. and Fontaine, G. and Funk, S. and Fuessling, M. and Gabici, S. and Gallant, Y. A. and Gate, F. and Giavitto, G. and Glawion, D. and Glicenstein, J. F. and Gottschall, D. and Grondin, M-H and Hahn, J. and Haupt, M. and Heinzelmann, G. and Henri, G. and Hermann, G. and Hinton, James Anthony and Hofmann, W. and Hoischen, Clemens and Holch, Tim Lukas and Holler, M. and Horns, D. and Huber, D. and Iwasaki, H. and Jacholkowska, A. and Jamrozy, M. and Jankowsky, D. and Jankowsky, F. and Jouvin, L. and Jung-Richardt, I and Kastendieck, M. A. and Katarzynski, K. and Katsuragawa, M. and Katz, U. and Khangulyan, D. and Khelifi, B. and King, J. and Klepser, S. and Kluzniak, W. and Komin, Nu and Kosack, K. and Kostunin, D. and Kraus, M. and Lamanna, G. and Lau, J. and Lemiere, A. and Lemoine-Goumard, M. and Lenain, J-P and Leser, Eva and Lohse, T. and Lopez-Coto, R. and Lypova, I and Malyshev, D. and Marandon, V and Marcowith, Alexandre and Mariaud, C. and Marti-Devesa, G. and Marx, R. and Maurin, G. and Maxted, N. and Meintjes, P. J. and Mitchell, A. M. W. and Moderski, R. and Mohamed, M. and Mohrmann, L. and Moore, C. and Moulin, Emmanuel and Murach, T. and Nakashima, S. and de Naurois, M. and Ndiyavala, H. and Niederwanger, F. and Niemiec, J. and Oakes, L. and Odaka, H. and Ohm, S. and Wilhelmi, E. de Ona and Ostrowski, M. and Oya, I and Panter, M. and Parsons, R. D. and Perennes, C. and Petrucci, P-O and Peyaud, B. and Piel, Q. and Pita, S. and Poireau, V and Noel, A. Priyana and Prokhorov, D. A. and Prokoph, H. and Puehlhofer, G. and Punch, M. and Quirrenbach, A. and Raab, S. and Rauth, R. and Reimer, A. and Reimer, O. and Renaud, M. and Rieger, F. and Rinchiuso, L. and Romoli, C. and Rowell, G. and Rudak, B. and Ruiz-Velasco, E. and Sahakian, V and Saito, S. and Sanchez, David M. and Santangelo, A. and Sasaki, M. and Schlickeiser, R. and Schussler, F. and Schulz, A. and Schutte, H. and Schwanke, U. and Schwemmer, S. and Seglar-Arroyo, M. and Senniappan, M. and Seyffert, A. S. and Shafi, N. and Shilon, I and Shiningayamwe, K. and Simoni, R. and Sinha, A. and Sol, H. and Specovius, A. and Spir-Jacob, M. and Stawarz, L. and Steenkamp, R. and Stegmann, Christian and Steppa, Constantin Beverly and Takahashi, T. and Tavernet, J-P and Tavernier, T. and Taylor, A. M. and Terrier, R. and Tibaldo, Luigi and Tiziani, D. and Tluczykont, M. and Trichard, C. and Tsirou, M. and Tsuji, N. and Tuffs, R. and Uchiyama, Y. and van der Walt, D. J. and van Eldik, C. and van Rensburg, C. and van Soelen, B. and Vasileiadis, G. and Veh, J. and Venter, C. and Vincent, P. and Vink, J. and Voisin, F. and Voelk, H. J. and Vuillaume, T. and Wadiasingh, Z. and Wagner, S. J. and White, R. and Wierzcholska, A. and Yang, R. and Yoneda, H. and Zaborov, D. and Zacharias, M. and Zanin, R. and Zdziarski, A. A. and Zech, Alraune and Ziegler, A. and Zorn, J. and Zywucka, N.}, title = {H.E.S.S. and Suzaku observations of the Vela X pulsar wind nebula}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {627}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, organization = {HESS Collaboration}, issn = {1432-0746}, doi = {10.1051/0004-6361/201935458}, pages = {16}, year = {2019}, abstract = {Context. Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon. Aims. Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling. Methods. We used data from the Suzaku XIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra. Results. The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10(-12) erg cm(-3). The data indicate the presence of a cutoff in the electron spectrum at energies of similar to 100 TeV and a magnetic field strength of similar to 6 mu G. Constraints on the presence of turbulent magnetic fields are weak. Conclusions. The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range greater than or similar to 10 keV.}, language = {en} }