@article{IrrgangGeierHeberetal.2019, author = {Irrgang, Andreas and Geier, Stephan Alfred and Heber, Ulrich and Kupfer, Thomas and F{\"u}rst, F.}, title = {PG 1610+062: a runaway B star challenging classical ejection mechanisms}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {628}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201935429}, pages = {17}, year = {2019}, abstract = {Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610+062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610+062 is a late B-type MS star of 4-5 M⊙ with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610+062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia's second data release, shows that PG 1610+062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of 550 ± 40 km s-1, which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known.}, language = {en} } @article{IrrgangGeierKreuzeretal.2020, author = {Irrgang, Andreas and Geier, Stephan and Kreuzer, Simon and Pelisoli, Ingrid Domingos and Heber, Ulrich}, title = {A stripped helium star in the potential black hole binary LB-1}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {633}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201937343}, pages = {9}, year = {2020}, abstract = {Context The recently claimed discovery of a massive (M-BH = 68(-13)(+11) M-circle dot) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution. Aims A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LSV +22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass. Methods To this end, we study the spectral energy distribution of LSV +22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe. Results Our analysis clearly shows that LSV +22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LSV +22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of 1.1 +/- 0.5 M-circle dot. Based on the binary system's mass function, this yields a minimum mass of 2-3 M-circle dot for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star. Conclusions The star LSV +22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object.}, 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{KrtičkaFeldmeier2021, author = {Krtička, Jiř{\´i} and Feldmeier, Achim}, title = {Stochastic light variations in hot stars from wind instability}, series = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, volume = {648}, journal = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/202040148}, pages = {9}, year = {2021}, abstract = {Context Line-driven wind instability is expected to cause small-scale wind inhomogeneities, X-ray emission, and wind line profile variability. The instability can already develop around the sonic point if it is initiated close to the photosphere due to stochastic turbulent motions. In such cases, it may leave its imprint on the light curve as a result of wind blanketing. Aims We study the photometric signatures of the line-driven wind instability. Methods We used line-driven wind instability simulations to determine the wind variability close to the star. We applied two types of boundary perturbations: a sinusoidal one that enables us to study in detail the development of the instability and a stochastic one given by a Langevin process that provides a more realistic boundary perturbation. We estimated the photometric variability from the resulting mass-flux variations. The variability was simulated assuming that the wind consists of a large number of independent conical wind sectors. We compared the simulated light curves with TESS light curves of OB stars that show stochastic variability. Results We find two typical signatures of line-driven wind instability in photometric data: a knee in the power spectrum of magnitude fluctuations, which appears due to engulfment of small-scale structure by larger structures, and a negative skewness of the distribution of fluctuations, which is the result of spatial dominance of rarefied regions. These features endure even when combining the light curves from independent wind sectors. Conclusions The stochastic photometric variability of OB stars bears certain signatures of the line-driven wind instability. The distribution function of observed photometric data shows negative skewness and the power spectra of a fraction of light curves exhibit a knee. This can be explained as a result of the line-driven wind instability triggered by stochastic base perturbations.}, language = {en} } @article{LetoTrigilioOskinovaetal.2018, author = {Leto, Paolo and Trigilio, C. and Oskinova, Lida and Ignace, R. and Buemi, C. S. and Umana, G. and Cavallaro, Francesco and Ingallinera, A. and Bufano, F. and Phillips, N. M. and Agliozzo, Claudia and Cerrigone, L. and Todt, Helge Tobias and Riggi, S. and Leone, Francesco}, title = {The polarization mode of the auroral radio emission from the early-type star HD 142301}, series = {Monthly notices of the Royal Astronomical Society}, volume = {482}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnrasl/sly179}, pages = {L4 -- L8}, year = {2018}, abstract = {We report the detection of the auroral radio emission from the early-type magnetic star HD142301. New VLA observations of HD142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense ( and vice versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD142301 magnetosphere.}, language = {en} } @article{LetoTrigilioOskinovaetal.2017, author = {Leto, Paolo and Trigilio, C. and Oskinova, Lida and Ignace, R. and Buemi, C. S. and Umana, G. and Ingallinera, A. and Todt, Helge Tobias and Leone, F.}, title = {The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae}, 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/stx267}, pages = {2820 -- 2833}, year = {2017}, abstract = {In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars.}, language = {en} } @article{LetoTrigilioOskinovaetal.2018, author = {Leto, Paolo and Trigilio, C. and Oskinova, Lida and Ignace, R. and Buemi, C. S. and Umana, G. and Ingallinera, A. and Leone, Francesco and Phillips, N. M. and Agliozzo, Claudia and Todt, Helge Tobias and Cerrigone, L.}, title = {A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907}, series = {Monthly notices of the Royal Astronomical Society}, volume = {476}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sty244}, pages = {562 -- 579}, year = {2018}, abstract = {We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.}, language = {en} } @article{LiermannHamannOskinova2012, author = {Liermann, A. and Hamann, Wolf-Rainer and Oskinova, Lida}, title = {The Quintuplet cluster III. Hertzsprung-Russell diagram and cluster age}, 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 = {10.1051/0004-6361/201117534}, pages = {9}, year = {2012}, abstract = {The Quintuplet, one of three massive stellar clusters in the Galactic center (GC), is located about 30 pc in projection from Sagittarius A*. We aim at the construction of the Hertzsprung-Russell diagram (HRD) of the cluster to study its evolution and to constrain its star-formation history. For this purpose we use the most complete spectral catalog of the Quintuplet stars. Based on the K-band spectra we determine stellar temperatures and luminosities for all stars in the catalog under the assumption of a uniform reddening towards the cluster. We find two groups in the resulting HRD: early-type OB stars and late-type KM stars, well separated from each other. By comparison with Geneva stellar evolution models we derive initial masses exceeding 8 M-circle dot for the OB stars. In the HRD these stars are located along an isochrone corresponding to an age of about 4 Myr. This confirms previous considerations, where a similar age estimate was based on the presence of evolved Wolf-Rayet stars in the cluster. We derive number ratios for the various spectral subtype groups (e.g. N-WR/N-O, N-WC/N-WN) and compare them with predictions of population synthesis models. We find that an instantaneous burst of star formation at about 3.3 to 3.6 Myr ago is the most likely scenario to form the Quintuplet cluster. Furthermore, we apply a mass-luminosity relation to construct the initial mass function (IMF) of the cluster. We find indications for a slightly top-heavy IMF. The late-type stars in the LHO catalog are red giant branch (RGB) stars or red supergiants (RSGs) according to their spectral signatures. Under the assumption that they are located at about the distance of the Galactic center we can derive their luminosities. The comparison with stellar evolution models reveals that the initial masses of these stars are lower than 15 M-circle dot implying that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to evolve into their present stage. It might be suspected that these late-type stars do not physically belong to the Quintuplet cluster. Indeed, most of them disqualify as cluster members because their radial velocities differ too much from the cluster average. Nevertheless, five of the brightest RGB/RSG stars from the LHO catalog share the mean radial velocity of the Quintuplet, and thus remain highly suspect for being gravitationally bound members. If so, this would challenge the cluster formation and evolution scenario.}, language = {en} } @misc{LiermannHamannOskinova2014, author = {Liermann, Angelika and Hamann, Wolf-Rainer and Oskinova, Lida}, title = {The quintuplet cluster III. Hertzsprung-Russell diagram and cluster age (vol 540, pg A14, 2012)}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {563}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117534e}, pages = {2}, year = {2014}, 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{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{NazeOskinovaGosset2013, author = {Naze, Yael and Oskinova, Lida and Gosset, Eric}, title = {A detailed x-ray investigation of zeta puppis - II. the variability on short and long timescales}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {763}, 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/763/2/143}, pages = {21}, year = {2013}, abstract = {Stellar winds are a crucial component of massive stars, but their exact properties still remain uncertain. To shed some light on this subject, we have analyzed an exceptional set of X-ray observations of zeta Puppis, one of the closest and brightest massive stars. The sensitive light curves that were derived reveal two major results. On the one hand, a slow modulation of the X-ray flux (with a relative amplitude of up to 15\% over 16 hr in the 0.3-4.0 keV band) is detected. Its characteristic timescale cannot be determined with precision, but amounts from one to several days. It could be related to corotating interaction regions, known to exist in zeta Puppis from UV observations. Hour-long changes, linked to flares or to the pulsation activity, are not observed in the last decade covered by the XMM observations; the 17 hr tentative period, previously reported in a ROSAT analysis, is not confirmed either and is thus transient, at best. On the other hand, short-term changes are surprisingly small (<1\% relative amplitude for the total energy band). In fact, they are compatible solely with the presence of Poisson noise in the data. This surprisingly low level of short-term variability, in view of the embedded wind-shock origin, requires a very high fragmentation of the stellar wind, for both absorbing and emitting features (>10(5) parcels, comparing with a two-dimensional wind model). This is the first time that constraints have been placed on the number of clumps in an O-type star wind and from X-ray observations.}, language = {en} } @article{PabloRichardsonMoffatetal.2015, author = {Pablo, Herbert and Richardson, Noel D. and Moffat, Anthony F. J. and Corcoran, Michael and Shenar, Tomer and Benvenuto, Omar and Fuller, Jim and Naze, Yael and Hoffman, Jennifer L. and Miroshnichenko, Anatoly and Apellaniz, Jesus Maiz and Evans, Nancy and Eversberg, Thomas and Gayley, Ken and Gull, Ted and Hamaguchi, Kenji and Hamann, Wolf-Rainer and Henrichs, Huib and Hole, Tabetha and Ignace, Richard and Iping, Rosina and Lauer, Jennifer and Leutenegger, Maurice and Lomax, Jamie and Nichols, Joy and Oskinova, Lida and Owocki, Stan and Pollock, Andy and Russell, Christopher M. P. and Waldron, Wayne and Buil, Christian and Garrel, Thierry and Graham, Keith and Heathcote, Bernard and Lemoult, Thierry and Li, Dong and Mauclaire, Benjamin and Potter, Mike and Ribeiro, Jose and Matthews, Jaymie and Cameron, Chris and Guenther, David and Kuschnig, Rainer and Rowe, Jason and Rucinski, Slavek and Sasselov, Dimitar and Weiss, Werner}, title = {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}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {809}, 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/809/2/134}, pages = {11}, year = {2015}, abstract = {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.}, 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{PillitteriFossatiRodriguezetal.2017, author = {Pillitteri, Ignazio and Fossati, Luca and Rodriguez, N. Castro and Oskinova, Lida and Wolk, Scott J.}, title = {Detection of magnetic field in the B2 star rho Ophiuchi A with ESO FORS2}, 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 = {10.1051/0004-6361/201732078}, pages = {4}, year = {2017}, abstract = {Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them (similar to 10\%) are strong and hard X-ray sources and possess strong magnetic fields. It is also known that some B-type stars have spots on their surface. Yet up to now no X-ray activity associated with spots on early-type stars was detected. In this Letter we report the detection of a magnetic field on the B2V star rho Oph A. Previously, we assessed that the X-ray activity of this star is associated with a surface spot, herewith we establish its magnetic origin. We analyze spectra of rho Oph A obtained with the FORS2 spectrograph at ESO Very Large Telescope (VLT) at two epochs, and detect a longitudinal component of the magnetic field of the order of similar to 500 G in one of the datasets. The detection of the magnetic field only at one epoch can be explained by stellar rotation which is also invoked to explain observed periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we derived the fundamental stellar parameters of rho Oph A and further constrained its age. We conclude that rho Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars.}, language = {en} } @article{PillitteriWolkRealeetal.2017, author = {Pillitteri, Ignazio and Wolk, Scott J. and Reale, Fabio and Oskinova, Lida}, title = {The early B-type star Rho Ophiuchi A is an X-ray lighthouse}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {602}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201630070}, pages = {10}, year = {2017}, abstract = {We present the results of a 140 ks XMM-Newton observation of the B2 star rho Oph A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK (kT similar to 5 keV). From the analysis of its rise, we infer a magnetic field of >= 300 G and a size of the flaring region of similar to 1.4-1.9 x 10(11) cm, which corresponds to similar to 25\%-30\% of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of >= 2.5 yr, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems. In both cases rho Oph would constitute an extreme system that is worthy of further investigation.}, 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{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{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{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} }