Sahai R, Sánchez Contreras C, Mangan A, Sanz-Forcada J, Muthumariappan C, Claussen M J
Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109.
Astrobiology Center (CSIC-INTA), ESAC campus, E-28691 Villanueva de la Cãnada, Madrid, Spain.
Astrophys J. 2018 Jun 18;860(2). doi: 10.3847/1538-4357/aac3d7.
Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the that show strong flickering in the UV continuum on time-scales of ≲ 20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blue-shifted by velocities of ~500 km s from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blue-shifted features due to absorption of UV continuum emited by the disk, whereas the red-shifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. (2015) that Y Gem's companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 , and thus a mass-accretion rate > 5 × 10 yr; we infer that Roche lobe overflow is the most likely binary accretion mode for Y Gem.
人们认为双星系统会极大地影响渐近巨星分支（AGB）和后渐近巨星分支恒星的质量损失历史和几何结构，但目前严重缺乏双星系统的观测证据。作为利用星系演化探测器（GALEX）档案库寻找低温AGB恒星的高温双星伴星项目的一部分，我们发现一颗M型晚期恒星——双子座Y星（Y Gem）是强烈且可变的紫外线和X射线发射源。在此，我们报告用[未提及的设备]获得的双子座Y星的紫外线光谱观测结果，这些结果显示紫外线连续谱在≲20秒的时间尺度上有强烈闪烁，这是活跃吸积盘的特征。还观测到了几条来自硅四价离子（Si IV）和碳四价离子（C IV）等物种的具有P - 仙王座型轮廓的紫外线谱线，其发射和吸收特征相对于系统速度有~500千米每秒的红移和蓝移。我们对这些（以及之前的）观测结果的模型是，来自主星的物质被伴星引力捕获，形成一个高温吸积盘。后者驱动一股快速外流，由于吸积盘发射的紫外线连续谱被吸收而产生蓝移特征，而红移发射特征则出现在来自主星的受热下落物质中。外流速度支持了萨海等人（2015年）之前的推断，即双子座Y星的伴星是一颗低质量主序星。对紫外线连续谱的黑体拟合意味着吸积光度约为13[未提及单位]，因此质量吸积率>5×10[未提及单位]年；我们推断洛希瓣溢流是双子座Y星最可能的双星吸积模式。
