TY  - JOUR
A1  - Ramachandran, Varsha
A1  - Hainich, Rainer
A1  - Hamann, Wolf-Rainer
A1  - Oskinova, Lida
A1  - Shenar, T.
A1  - Sander, Andreas Alexander Christoph
A1  - Todt, Helge Tobias
A1  - Gallagher, John S.
T1  - Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - 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.
KW  - stars: early-type
KW  - Magellanic Clouds
KW  - stars: atmospheres
KW  - stars: winds, outflows
KW  - stars: mass-loss
KW  - stars: massive
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201731093
SN  - 1432-0746
SN  - 0004-6361
VL  - 609
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Pena, M.
A1  - Hamann, Wolf-Rainer
A1  - Ruiz, M. T.
T1  - The LMC planetary nebula N66 revisited. Nebular kinematics and stellar models
Y1  - 2003
SN  - 1-583-81148-6
ER  - 
TY  - JOUR
A1  - Pena, M.
A1  - Hamann, Wolf-Rainer
A1  - Koesterke, Lars
A1  - Maza, J.
A1  - Mendez, R. H.
A1  - Peimbert, M.
A1  - Ruiz, M. T.
A1  - Torres-Peimbert, S.
T1  - HST spectrophotometric data of the central star of the planetary nebula LMC-N66
Y1  - 1997
ER  - 
TY  - JOUR
A1  - Pena, M.
A1  - Hamann, Wolf-Rainer
A1  - Koesterke, Lars
A1  - Maza, J.
A1  - Mendez, R. H.
A1  - Peimbert, M.
A1  - Ruiz, M. T.
A1  - Torres-Peimbert, S.
T1  - Spectrophotometric data of the central star of the large magellanic cloud planetary nebula N66. Quantitative analysis of its WN type spectrum
Y1  - 1997
ER  - 
TY  - JOUR
A1  - Pena, M.
A1  - Hamann, Wolf-Rainer
T1  - The central star of the planetary nebula LMC-N66 : a massive accreting white dwarf?
N2  - The central star of the PN LMC-N66 showed an impressive outburst in 1993 - 1994, returning to its initial conditions about 8 years later. Its spectrum resembles that of a WN4.5 star, being the only confirmed central star of planetary nebulae showing such a spectral type. Recent analysis for the central star parameters, performed by Hamann et al. (2003) is presented. They have found that the bolometric luminosity increased by a factor larger than 6, during the outburst. We discuss the possible scenarios which have been proposed to explain the exceptional stellar parameters and the outburst mechanism. The stellar characteristics and the morphology and kinematics of the planetary nebula suggest the presence of binary system (massive star with a less massive companion or, a white dwarf accreting matter in a close- binary system). These cases pose the least severe contradictions with observational constraints.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Pena, M
A1  - Peimbert, A.
A1  - Hamann, Wolf-Rainer
A1  - Ruiz, M. T.
A1  - Peimbert, M.
T1  - The extraordinary planetary nebula N66 in the LMC
N2  - Morphology of the planetary nebula LMC-N66 (ionized by a [WN] star) indicates that the nebula is a multipolar object with a very narrow waist. It shows several jets, knots and filaments in opposite directions from the central star. A couple of twisted long filaments could be interpreted as due to point-symmetric type ejection. If such is the case, the progenitor would be a binary precessing system. High resolution spectroscopy shows that most of the material is approaching or receding from the star. However the line profiles are very complex, showing several components at different velocities. Our high resolution spectroscopic data show that the different structures (knots, filaments, ...) present different radial velocities spreading from 240 to more than 400 km/s. The system velocity is 300 km/s. There are high velocity knots located to the north of the central star, moving at more than 100 km/s relative to the system velocity.
Y1  - 2004
SN  - 3-12-283174-0
ER  - 
TY  - JOUR
A1  - Pablo, Herbert
A1  - Richardson, Noel D.
A1  - Moffat, Anthony F. J.
A1  - Corcoran, Michael
A1  - Shenar, Tomer
A1  - Benvenuto, Omar
A1  - Fuller, Jim
A1  - Naze, Yael
A1  - Hoffman, Jennifer L.
A1  - Miroshnichenko, Anatoly
A1  - Apellaniz, Jesus Maiz
A1  - Evans, Nancy
A1  - Eversberg, Thomas
A1  - Gayley, Ken
A1  - Gull, Ted
A1  - Hamaguchi, Kenji
A1  - Hamann, Wolf-Rainer
A1  - Henrichs, Huib
A1  - Hole, Tabetha
A1  - Ignace, Richard
A1  - Iping, Rosina
A1  - Lauer, Jennifer
A1  - Leutenegger, Maurice
A1  - Lomax, Jamie
A1  - Nichols, Joy
A1  - Oskinova, Lida
A1  - Owocki, Stan
A1  - Pollock, Andy
A1  - Russell, Christopher M. P.
A1  - Waldron, Wayne
A1  - Buil, Christian
A1  - Garrel, Thierry
A1  - Graham, Keith
A1  - Heathcote, Bernard
A1  - Lemoult, Thierry
A1  - Li, Dong
A1  - Mauclaire, Benjamin
A1  - Potter, Mike
A1  - Ribeiro, Jose
A1  - Matthews, Jaymie
A1  - Cameron, Chris
A1  - Guenther, David
A1  - Kuschnig, Rainer
A1  - Rowe, Jason
A1  - Rucinski, Slavek
A1  - Sasselov, Dimitar
A1  - Weiss, Werner
T1  - A coordinated X-Ray and optical campaign of the nearest massive eclipsing binary, delta ORIONIS Aa. III. Analysis of optical photometric (most) and spectroscopic (ground based) variations
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We report on both high-precision photometry from the Microvariability and Oscillations of Stars (MOST) space telescope and ground-based spectroscopy of the triple system delta Ori A, consisting of a binary O9.5II+early-B (Aa1 and Aa2) with P = 5.7 days, and a more distant tertiary (O9 IV P > 400 years). This data was collected in concert with X-ray spectroscopy from the Chandra X-ray Observatory. Thanks to continuous coverage for three weeks, the MOST light curve reveals clear eclipses between Aa1 and Aa2 for the first time in non-phased data. From the spectroscopy, we have a well-constrained radial velocity (RV) curve of Aa1. While we are unable to recover RV variations of the secondary star, we are able to constrain several fundamental parameters of this system and determine an approximate mass of the primary using apsidal motion. We also detected second order modulations at 12 separate frequencies with spacings indicative of tidally influenced oscillations. These spacings have never been seen in a massive binary, making this system one of only a handful of such binaries that show evidence for tidally induced pulsations.
KW  - binaries: close
KW  - binaries: eclipsing
KW  - stars: early-type
KW  - stars: individual (delta Ori A)
KW  - stars: mass-loss
KW  - stars: variables: general
Y1  - 2015
U6  - https://doi.org/10.1088/0004-637X/809/2/134
SN  - 0004-637X
SN  - 1538-4357
VL  - 809
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Todt, Helge Tobias
A1  - Ignace, Richard
A1  - Brown, John C.
A1  - Cassinelli, Joseph P.
A1  - Hamann, Wolf-Rainer
T1  - Early magnetic B-type stars X-ray emission and wind properties
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present a comprehensive study of X-ray emission by, and wind properties of, massive magnetic early B-type stars. Dedicated XMM-Newton observations were obtained for three early-type B-type stars, xi(1) CMa, V2052 Oph and zeta Cas, with recently discovered magnetic fields. We report the first detection of X-ray emission from V2052 Oph and zeta Cas. The latter is one the softest X-ray sources among the early-type stars, while the former is one of the X-ray faintest. The observations show that the X-ray spectra of our programme stars are quite soft with the bulk of X-ray emitting material having a temperature of about 1 MK. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that the X-ray properties of early massive B-type magnetic stars are diverse, and that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field, corroborating similar conclusions reached earlier for the classical chemically peculiar magnetic Bp-Ap stars.
 We analyse the ultraviolet (UV) spectra of five non-supergiant B stars with magnetic fields (tau Sco, beta Cep, xi(1) CMa, V2052 Oph and zeta Cas) by means of non-local thermodynamic equilibrium (non-LTE) iron-blanketed model atmospheres. The latter are calculated with the Potsdam Wolf-Rayet (PoWR) code, which treats the photosphere as well as the wind, and also accounts for X-rays. With the exception of t Sco, this is the first analysis of these stars by means of stellar wind models. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high.
 Our analysis reveals that the magnetic early-type B stars studied here have weak winds with velocities not significantly exceeding upsilon(esc). The mass-loss rates inferred from the analysis of UV lines are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive a stronger mass-loss than we empirically infer from the UV spectral lines.
KW  - techniques: spectroscopic
KW  - stars: magnetic field
KW  - stars: massive
KW  - stars: mass loss
KW  - X-rays: stars
Y1  - 2011
U6  - https://doi.org/10.1111/j.1365-2966.2011.19143.x
SN  - 0035-8711
VL  - 416
IS  - 2
SP  - 1456
EP  - 1474
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Todt, Helge Tobias
A1  - Huenemoerder, David P.
A1  - Hubrig, Swetlana
A1  - Ignace, Richard
A1  - Hamann, Wolf-Rainer
A1  - Balona, Luis
T1  - On X-ray pulsations in beta Cephei-type variables
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays.
 Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars.
 Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity.
 Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question.
KW  - stars: massive
KW  - stars: variables: general
KW  - X-rays: stars
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201525908
SN  - 0004-6361
SN  - 1432-0746
VL  - 577
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Naze, Yael
A1  - Todt, Helge Tobias
A1  - Huenemoerder, David P.
A1  - Ignace, Richard
A1  - Hubrig, Swetlana
A1  - Hamann, Wolf-Rainer
T1  - Discovery of X-ray pulsations from a massive star
JF  - Nature Communications
N2  - X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. All X-ray pulsars known so far are associated with degenerate objects, either neutron stars or white dwarfs. Here we report the discovery of pulsating X-rays from a non-degenerate object, the massive B-type star xi(1) CMa. This star is a variable of beta Cep-type and has a strong magnetic field. Our observations with the X-ray Multi-Mirror (XMM-Newton) telescope reveal X-ray pulsations with the same period as the fundamental stellar oscillations. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission and their magnetism.
Y1  - 2014
U6  - https://doi.org/10.1038/ncomms5024
SN  - 2041-1723
VL  - 5
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Steinke, M.
A1  - Hamann, Wolf-Rainer
A1  - Sander, A.
A1  - Todt, Helge Tobias
A1  - Liermann, Adriane
T1  - One of the most massive stars in the Galaxy may have formed in isolation
JF  - Monthly notices of the Royal Astronomical Society
N2  - Very massive stars, 100 times heavier than the sun, are rare. It is not yet known whether such stars can form in isolation or only in star clusters. The answer to this question is of fundamental importance. The central region of our Galaxy is ideal for investigating very massive stars and clusters located in the same environment. We used archival infrared images to investigate the surroundings of apparently isolated massive stars presently known in the Galactic Centre (GC). We find that two such isolated massive stars display bow shocks and hence may be 'runaways' from their birthplace. Thus, some isolated massive stars in the GC region might have been born in star clusters known in this region. However, no bow shock is detected around the isolated star WR 102ka (Peony nebula star), which is one of the most massive and luminous stars in the Galaxy. This star is located at the centre of an associated circumstellar nebula. To study whether a star cluster may be 'hidden' in the surroundings of WR 102ka, to obtain new and better spectra of this star, and to measure its radial velocity, we obtained observations with the integral-field spectrograph SINFONI at the ESO's Very Large Telescope. Our observations confirm that WR 102ka is one of the most massive stars in the Galaxy and reveal that this star is not associated with a star cluster. We suggest that WR 102ka has been born in relative isolation, outside of any massive star cluster.
KW  - stars: individual: WR 102ka
KW  - Galaxy: centre
KW  - infrared: stars
Y1  - 2013
U6  - https://doi.org/10.1093/mnras/stt1817
SN  - 0035-8711
SN  - 1365-2966
VL  - 436
IS  - 4
SP  - 3357
EP  - 3365
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Kubatova, Brankica
A1  - Hamann, Wolf-Rainer
T1  - Moving inhomogeneous envelopes of stars
JF  - Transport in Porous Media
N2  - Massive stars are extremely luminous and drive strong winds, blowing a large part of their matter into the galactic environment before they finally explode as a supernova. Quantitative knowledge of massive star feedback is required to understand our Universe as we see it. Traditionally, massive stars have been studied under the assumption that their winds are homogeneous and stationary, largely relying on the Sobolev approximation. However, Observations with the newest instruments, together with progress in model calculations, ultimately dictate a cardinal change of this paradigm: stellar winds are highly inhomogeneous. Hence, we are now advancing to a new stage in our understanding of stellar winds. Using the foundations laid by V.V. Sobolev and his school, we now update and further develop the stellar spectral analysis techniques. New sophisticated 3-D models of radiation transfer in inhomogeneous expanding media elucidate the physics of stellar winds and improve classical empiric mass-loss rate diagnostics. Applications of these new techniques to multiwavelength observations of massive stars yield consistent and robust stellar wind parameters. (C) 2016 Elsevier Ltd. All rights reserved.
KW  - Stars: mass-loss
KW  - Stars: winds
KW  - Outflows
KW  - Stars: atmospheres early type
Y1  - 2016
U6  - https://doi.org/10.1016/j.jqsrt.2016.06.017
SN  - 0022-4073
SN  - 1879-1352
VL  - 183
SP  - 100
EP  - 112
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Ignace, Richard
A1  - Hamann, Wolf-Rainer
A1  - Pollock, A. M. T.
A1  - Brown, John C.
T1  - The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
N2  - The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of Lx < 2.5 x 1030 ergs-1 and to the X-ray to bolometric luminosity ratio of Lx/Lbol < 4 x 10-9. This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and CHANDRA observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven stellar winds of this particular class are insignificant X-ray sources. We attribute this to photoelectronic absorption by the stellar wind. The high opacity of the metal-rich and dense winds from WC stars puts the radius of optical depth unity at hundreds or thousands of stellar radii for much of the X-ray band. We believe that the essential absence of hot plasma so far out in the wind exacerbated by the large distances and correspondingly high ISM column densities makes the WC stars too faint to be detectable with current technology. The result also applies to many WC stars in binary systems, of which only about 20 % are identified X-ray sources, presumably due to colliding winds.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Huenemoerder, D. P.
A1  - Hamann, Wolf-Rainer
A1  - Shenar, Tomer
A1  - Sander, Andreas Alexander Christoph
A1  - Ignace, R.
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
T1  - On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.
KW  - stars: individual (Cyg OB2 12)
KW  - stars: massive
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - supergiants
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.3847/1538-4357/aa7e79
SN  - 0004-637X
SN  - 1538-4357
VL  - 845
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Hamann, Wolf-Rainer
A1  - Feldmeier, Achim
A1  - Ignace, Richard
A1  - Chu, You-Hua
T1  - Discovery of X-ray emission from the Wolf-Rayet star WR 142 of oxygen subtype
N2  - We report the discovery of weak yet hard X-ray emission from the Wolf-Rayet (WR) star WR 142 with the XMM- Newton X-ray telescope. Being of spectral subtype WO2, WR 142 is a massive star in a very advanced evolutionary stage shortly before its explosion as a supernova or. gamma-ray burst. This is the first detection of X-ray emission from a WO- type star. We rule out any serendipitous X-ray sources within approximate to 1 '' of WR 142. WR 142 has an X- ray luminosity of L-X approximate to 7 x 10(30) erg s(-1), which constitutes only less than or similar to 10(-8) of its bolometric luminosity. The hard X-ray spectrum suggests a plasma temperature of about 100 MK. Commonly, X-ray emission from stellar winds is attributed to embedded shocks due to the intrinsic instability of the radiation driving. From qualitative considerations we conclude that this mechanism cannot account for the hardness of the observed radiation. There are no hints for a binary companion. Therefore the only remaining, albeit speculative explanation must refer to magnetic activity. Possibly related, WR 142 seems to rotate extremely fast, as indicated by the unusually round profiles of its optical emission lines. Our detection implies that the wind of WR 142 must be relatively transparent to X-rays, which can be due to strong wind ionization, wind clumping, or nonspherical geometry from rapid rotation.
Y1  - 2009
UR  - http://iopscience.iop.org/0004-637X/
U6  - https://doi.org/10.1088/0004-637x/693/1/L44
SN  - 0004-637X
ER  - 
TY  - CHAP
A1  - Oskinova, Lida
A1  - Hamann, Wolf-Rainer
A1  - Feldmeier, Achim
T1  - X-raying clumped stellar winds
N2  - X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. Generally, to correctly model the emerging Xray spectra, a detailed knowledge of the cool wind opacities based on stellar atmosphere models is prerequisite. A nearly grey stellar wind opacity for the X-rays is deduced from the analyses of high-resolution X-ray spectra. This indicates that the stellar winds are strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission line profiles can be explained if the wind clumps are radially compressed. In massive binaries the orbital variations of X-ray emission allow to probe the opacity of the stellar wind; results support the picture of strong wind clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the extend of the stellar-wind part photoionized by X-rays provide further strong evidence that stellar winds consist of dense clumps.
Y1  - 2007
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-18133
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Hamann, Wolf-Rainer
A1  - Cassinelli, Joseph P.
A1  - Brown, John C.
A1  - Todt, Helge Tobias
T1  - X-ray emission from massive stars with magnetic fields
JF  - Astronomische Nachrichten = Astronomical notes
N2  - We investigate the connections between the magnetic fields and the X-ray emission from massive stars. Our study shows that the X-ray properties of known strongly magnetic stars are diverse: while some comply to the predictions of the magnetically confined wind model, others do not. We conclude that strong, hard, and variable X-ray emission may be a sufficient attribute of magnetic massive stars, but it is not a necessary one. We address the general properties of X-ray emission from "normal" massive stars, especially the long standing mystery about the correlations between the parameters of X-ray emission and fundamental stellar properties. The recent development in stellar structure modeling shows that small-scale surface magnetic fields may be common. We suggest a "hybrid" scenario which could explain the X-ray emission from massive stars by a combination of magnetic mechanisms on the surface and shocks in the stellar wind. The magnetic mechanisms and the wind shocks are triggered by convective motions in sub-photospheric layers. This scenario opens the door for a natural explanation of the well established correlation between bolometric and X-ray luminosities.
KW  - stars: magnetic fields
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: Wolf-Rayet
KW  - techniques: spectroscopic
KW  - X-rays: stars
Y1  - 2011
U6  - https://doi.org/10.1002/asna.201111602
SN  - 0004-6337
VL  - 332
IS  - 9-10
SP  - 988
EP  - 993
PB  - Wiley-Blackwell
CY  - Malden
ER  - 
TY  - GEN
A1  - Oskinova, Lida
A1  - Gayley, K. G.
A1  - Hamann, Wolf-Rainer
A1  - Hünemörder, D. P.
A1  - Ignace, R.
A1  - Pollock, A. M. T.
T1  - High-Resolution X-Ray Spectroscopy reveals the special nature of Wolf-Rayet star winds (pg 747, 2012)
T2  - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters
Y1  - 2012
U6  - https://doi.org/10.1088/2041-8205/752/2/L35
SN  - 2041-8205
VL  - 752
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Feldmeier, Achim
A1  - Hamann, Wolf-Rainer
T1  - X-ray emission lines from inhomogeneous stellar winds
N2  - It is commonly adopted that X-rays from O stars are produced deep inside the stellar wind, and transported outwards through the bulk of the expanding matter which attenuates the radiation and affects the shape of emission line profiles. The ability of the X-ray observatories Chandra and XMM-Newton to resolve these lines spectroscopically provided a stringent test for the theory of the X-ray production. It turned out that none of the existing models was able to fit the observations consistently. The possible caveat of these models was the underlying assumption of a smooth stellar wind. Motivated by the evidence that the stellar winds are in fact structured, we present a 2-D numerical model of a stochastic, inhomogeneous wind. Small parcels of hot, X-ray emitting gas are permeated by cool, absorbing wind material which is compressed into thin shell fragments. Wind fragmentation alters the radiative transfer drastically, compared to homogeneous models of the same mass-loss rate. X-rays produced deep inside the wind, which would be totally absorbed in a homogeneous flow, can effectively escape from a fragmented wind. The wind absorption becomes wavelength independent if the individual fragments are optically thick. The X-ray line profiles are flat-topped in the blue part and decline steeply in the red part for the winds with a short acceleration zone. For the winds where the acceleration extends over significant distances, the lines can appear nearly symmetric and only slightly blueshifted, in contrast to the skewed, triangular line profiles typically obtained from homogeneous wind models of high optical depth. We show that profiles from a fragmented wind model can reproduce the observed line profiles from zeta Orionis. The present numerical modeling confirms the results from a previous study, where we derived analytical formulae from a statistical treatment
Y1  - 2004
SN  - 0004-6361
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Feldmeier, Achim
A1  - Hamann, Wolf-Rainer
T1  - X-ray line profiles from structured stellar winds
N2  - Absorbing material compressed in a number of thin shells is effectively less opaque for X-rays than smoothly distributed gas. The calculated X-ray emission line profiles are red-shifted if the emission arises from the starward side of the shells.
Y1  - 2003
SN  - 1-58381-133-8
ER  -