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arXiv 1503.00930astro-ph.SR
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VLT-FLAMES狼蛛调查XX。X射线亮发射线星VFTS 399的性质

The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission line star VFTS 399

J. S. Clark, E. S. Bartlett, P. S. Broos, L. K. Townsley, W. D. Taylor, N. R. Walborn, A. J. Bird, H. Sana, S. E. de Mink, P. L. Dufton, C. J. Evans, N. Langer,… 展开作者

J. S. Clark, E. S. Bartlett, P. S. Broos, L. K. Townsley, W. D. Taylor, N. R. Walborn, A. J. Bird, H. Sana, S. E. de Mink, P. L. Dufton, C. J. Evans, N. Langer, J. Maíz Apellániz, F. R. N. Schneider, I. Soszyński

摘要:

在大麦云星云中的剑鱼座30恒星形成区的恒星星族包含了一个明显的单颗快速旋转的O型恒星的子集。导致这一群体形成的物理过程目前尚不确定。这群中的一个成员,晚O型星VFTS 399,被发现在它的热光度中出人意料地具有X射线亮度-在这项研究中,我们的目标是确定它的物理性质和这种行为的原因。我们发现VFTS 399是一个非周期光度变量,具有明显的近红外过剩。它的光谱显示了较低的巴耳末系和选择的HeI谱线的复杂发射轮廓,这些结合在一起表明了OeBe分类。X射线的光度变化很大,不可能由一颗恒星产生,而其坚硬的非热性质表明存在一个吸积的相对论伴星。最后,X射线光变曲线的周期性调制的检测是最自然的假设下,吸积体是一个中子星的解释。VFTS 399似乎是第一个在30 Dor范围内发现的高质量X射线双星。比较VFTS 399与双星演化模型的当前性质,推测中子星的前身质量超过25 Msun,可能拥有与磁星强度相当的磁场。VFTS 399现在是30 Dor中快速旋转的“单一”O型恒星群中的第二个成员,显示出导致自旋上升的二元相互作用的证据,这表明这可能是形成这个恒星星族子集的可行进化途径。

英文摘要:

The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity - in this study we aim to determine its physical nature and the cause of this behaviour. We find VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select HeI lines - taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass in excess of 25Msun for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating `single' O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population.

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