Spectroscopic twin to the hypervelocity sdO star US 708 and three fast sdB stars from the Hyper-MUCHFUSS project
Spectroscopic twin to the hypervelocity sdO star US 708 and three fast sdB stars from the Hyper-MUCHFUSS project
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DOI：
10.1051/0004-6361/201730437
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发表时间：
2017-03
影响因子：
6.5
通讯作者：
E. Ziegerer;U. Heber;S. Geier;A. Irrgang;T. Kupfer;F. Furst;J. Schaffenroth
中科院分区：
文献类型：
--
作者：
E. Ziegerer;U. Heber;S. Geier;A. Irrgang;T. Kupfer;F. Furst;J. Schaffenroth
银河系暗物质晕的重要示踪剂是超高速恒星（HVSs），它们比银河系的局部逃逸速度更快，也比它们的慢速同行——银河系晕中的高速恒星更快。这样的HVSs被认为是超大质量黑洞（Hills机制）通过对双星的潮汐破坏从银河系中心（GC）喷射出来的。Hyper-MUCHFUSS的目的是寻找高速的、可能脱离束缚的热亚矮星。我们利用光学Keck/ESI和VLT （x -射手，FORS）光谱对一颗He-sdO以及sdB恒星中的三颗候选恒星进行了光谱和运动学分析。适当的运动是通过结合早期摄影板的位置和现代数字巡天的位置来确定的。银河系静止坐标系的速度范围从203 km s^（-1）到660 km s^(-1)，这表明很可能所有四颗恒星都被引力束缚在银河系中。SDSS J205030.39−061957.8 （J2050）是超高速He-sdO US 708的光谱孪生体，其T_(eff) = 47000 K，表面重力为log g = 5.7。对于后者，根据运动学分析排除了GC作为原点。因此，J2050可以排除希尔斯机制。弹射速度（385±79 km s^(-1)）比US 708（998±68 km s^(-1)）温和得多。对于US 708提出的双热核超新星情景将很好地解释J2050的观测特性，而不会像US 708所要求的那样将模型参数推到极限。因此，这颗恒星可能是幸存下来的Ia型超新星的供体。3颗sdB星也表现出极端的运动学；一个可能是从气相星系喷出的HVS，而另外两个可能是通过双超新星机制从银盘喷出的。或者，它们可能是极端晕星。
Important tracers for the dark matter halo of the Galaxy are hypervelocity stars (HVSs), which are faster than the local escape velocity of the Galaxy and their slower counterparts, the high-velocity stars in the Galactic halo. Such HVSs are believed to be ejected from the Galactic centre (GC) through tidal disruption of a binary by the super-massive black hole (Hills mechanism). The Hyper-MUCHFUSS survey aims at finding high-velocity potentially unbound hot subdwarf stars. We present the spectroscopic and kinematical analyses of a He-sdO as well as three candidates among the sdB stars using optical Keck/ESI and VLT (X-shooter, FORS) spectroscopy. Proper motions are determined by combining positions from early-epoch photographic plates with those derived from modern digital sky surveys. The Galactic rest frame velocities range from 203 km s^(-1) to 660 km s^(-1), indicating that most likely all four stars are gravitationally bound to the Galaxy. With T_(eff) = 47 000 K and a surface gravity of log g = 5.7, SDSS J205030.39−061957.8 (J2050) is a spectroscopic twin of the hypervelocity He-sdO US 708. As for the latter, the GC is excluded as a place of origin based on the kinematic analysis. Hence, the Hills mechanism can be excluded for J2050. The ejection velocity is much more moderate (385 ± 79 km s^(-1)) than that of US 708 (998 ± 68 km s^(-1)). The binary thermonuclear supernova scenario suggested for US 708 would explain the observed properties of J2050 very well without pushing the model parameters to their extreme limits, as required for US 708. Accordingly, the star would be the surviving donor of a type Ia supernova. Three sdB stars also showed extreme kinematics; one could be a HVS ejected from the GC, whereas the other two could be ejected from the Galactic disk through the binary supernova mechanism. Alternatively, they might be extreme halo stars.