TY - JOUR A1 - Surlan, B. A1 - Hamann, Wolf-Rainer A1 - Kubat, Jirij A1 - Oskinova, Lida A1 - Feldmeier, Achim T1 - Three-dimensional radiative transfer in clumped hot star winds I influence of clumping on the resonance line formation JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - stars: winds, outflows KW - stars: mass-loss KW - stars: early-type Y1 - 2012 U6 - https://doi.org/10.1051/0004-6361/201118590 SN - 0004-6361 VL - 541 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Oskinova, Lida A1 - Gayley, K. G. A1 - Hamann, Wolf-Rainer A1 - Huenemoerder, 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 JF - ASTROPHYSICAL JOURNAL LETTERS N2 - 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. KW - stars: individual (WR 6) KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - X-rays: stars Y1 - 2012 U6 - https://doi.org/10.1088/2041-8205/747/2/L25 SN - 2041-8205 VL - 747 IS - 2 PB - IOP PUBLISHING LTD CY - BRISTOL ER - TY - JOUR A1 - Guerrero, Martín A. A1 - Ruiz, N. A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Todt, Helge Tobias A1 - Schönberner, Detlef A1 - Oskinova, Lida A1 - Gründl, R. A. A1 - Steffen, M. A1 - Blair, William P. A1 - Toala, Jesús Alberto T1 - Rebirth of X-Ray emission from the born-again planetary Nebula A30 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible. KW - planetary nebulae: general KW - planetary nebulae: individual (A30) KW - stars: winds, outflows KW - X-rays: ISM Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/755/2/129 SN - 0004-637X VL - 755 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Parkin, E. R. A1 - Broos, Patrick S. A1 - Townsley, L. K. A1 - Pittard, J. M. A1 - Moffat, Anthony F. J. A1 - Naze, Y. A1 - Rauw, G. A1 - Oskinova, Lida A1 - Waldron, W. L. T1 - X-RAY EMISSION FROM THE DOUBLE-BINARY OB-STAR SYSTEM QZ CAR (HD 93206) JF - ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES N2 - 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. KW - hydrodynamics KW - stars: early-type KW - stars: individual (QZ Carinae) KW - stars: massive KW - stars: winds, outflows KW - X-rays: stars Y1 - 2011 U6 - https://doi.org/10.1088/0067-0049/194/1/8 SN - 0067-0049 VL - 194 IS - 1 PB - IOP PUBLISHING LTD CY - BRISTOL ER - TY - JOUR A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Cassinelli, Joseph P. A1 - Brown, John C. A1 - Todt, Helge Tobias T1 - X-ray emission from massive stars with magnetic fields JF - Astronomische Nachrichten = Astronomical notes N2 - We investigate the connections between the magnetic fields and the X-ray emission from massive stars. Our study shows that the X-ray properties of known strongly magnetic stars are diverse: while some comply to the predictions of the magnetically confined wind model, others do not. We conclude that strong, hard, and variable X-ray emission may be a sufficient attribute of magnetic massive stars, but it is not a necessary one. We address the general properties of X-ray emission from "normal" massive stars, especially the long standing mystery about the correlations between the parameters of X-ray emission and fundamental stellar properties. The recent development in stellar structure modeling shows that small-scale surface magnetic fields may be common. We suggest a "hybrid" scenario which could explain the X-ray emission from massive stars by a combination of magnetic mechanisms on the surface and shocks in the stellar wind. The magnetic mechanisms and the wind shocks are triggered by convective motions in sub-photospheric layers. This scenario opens the door for a natural explanation of the well established correlation between bolometric and X-ray luminosities. KW - stars: magnetic fields KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - techniques: spectroscopic KW - X-rays: stars Y1 - 2011 U6 - https://doi.org/10.1002/asna.201111602 SN - 0004-6337 VL - 332 IS - 9-10 SP - 988 EP - 993 PB - Wiley-Blackwell CY - Malden ER -