TY - JOUR A1 - Huenemoerder, D. A1 - Gayley, K. A1 - Hamann, Wolf-Rainer A1 - Ignace, R. A1 - Nichols, J. A1 - Oskinova, Lida A1 - Pollock, A. M. T. A1 - Schulz, N. T1 - High Resolution X-Ray Spectra of WR 6 JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - As WR 6 is a putatively single WN4 star, and is relatively bright (V = 6.9), it is an ideal case for studying the wind mechanisms in these extremely luminous stars. To obtain higher resolution spectra at higher energy (above 1 keV) than previously obtained with the XMM/Newton RGS, we have observed WR 6 with the Chandra High Energy Transmission Grating Spectrometer for 450 ks. We have resolved emission lines of S, Si, Mg, Ne, and Fe, which all show a “fin"-shaped prole, characteristic of a self-absorbed uniformly expanding shell. Steep blue edges gives robust maximal expansion velocities of about 2000 km/s, somewhat larger than the 1700km/s derived from UV lines. The He-like lines all indicate that X-ray emitting plasmas are far from the photosphere – even at the higher energies where opacity is lowest { as was also the case for the longer wavelength lines observed with XMM-Newton/RGS. Abundances determined from X-ray spectral modeling indicate enhancements consistent with nucleosynthesis. The star was also variable in X-rays and in simultaneous optical photometry obtained with Chandra aspect camera, but not coherently with the optically known period of 3.765 days. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88236 SP - 301 EP - 304 ER - TY - JOUR A1 - Huenemoerder, David P. A1 - Gayley, K. G. A1 - Hamann, Wolf-Rainer A1 - Ignace, R. A1 - Nichols, J. S. A1 - Oskinova, Lida A1 - Pollock, A. M. T. A1 - Schulz, Norbert S. A1 - Shenar, Tomer T1 - Probing Wolf-Rayet winds: Chandra/HETG X-ray spectra of WR 6 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear-processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars. KW - stars: individual (WR 6) KW - stars: massive KW - stars: Wolf-Rayet Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/815/1/29 SN - 0004-637X SN - 1538-4357 VL - 815 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - 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 - 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 - 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 - Russell, C. M. P. A1 - Corcoran, M. F. A1 - Cuadra, J. A1 - Owocki, S. P. A1 - Wang, Q. D. A1 - Hamaguchi, K. A1 - Sugawara, Y. A1 - Pollock, A. M. T. A1 - Kallman, T. R. T1 - Hydrodynamic and radiative transfer modeling of X-ray emission from colliding WR winds BT - WR 140 & the Galactic center JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - Colliding Wolf-Rayet (WR) winds produce thermal X-ray emission widely observed by X-ray telescopes. In wide WR+O binaries, such as WR 140, the X-ray flux is tied to the orbital phase, and is a direct probe of the winds’ properties. In the Galactic center, ~30 WRs orbit the super massive black hole (SMBH) within ~10”, leading to a smorgasbord of wind-wind collisions. To model the X-ray emission of WR 140 and the Galactic center, we perform 3D hydrodynamic simulations to trace the complex gaseous flows, and then carry out 3D radiative transfer calculations to compute the variable X-ray spectra. The model WR 140 RXTE light curve matches the data well for all phases except the X-ray minimum associated with periastron, while the model spectra agree with the RXTE hardness ratio and the shape of the Suzaku observations throughout the orbit. The Galactic center model of the Chandra flux and spectral shape match well in the region r ≤ 3”, but the model flux falls off too rapidly beyond this radius. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88255 SP - 309 EP - 312 ER - TY - JOUR A1 - Sugawara, Y. A1 - Tsuboi, Y. A1 - Maeda, Y. A1 - Pollock, A. M. T. A1 - Williams, P. M. T1 - The Swift monitoring of the colliding wind binary WR 21a JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - The X-ray observations of the colliding wind binary WR 21a is reported. The first monitoring performed by Swift/XRT in order to reveal the phase-locked variation. Our observations cover 201 different epochs from 2013 October 1 to 2015 January 30 for a total exposure of about 306 ks. It is found for the first time that the luminosity varies roughly in inverse proportion to the separation of the two stars before the X-ray maximum but later drops rapidly toward periastron. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88517 SP - 366 ER -