The binary fraction of extreme horizontal branch stars
The binary fraction of extreme horizontal branch stars
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DOI：
10.1111/j.1365-2966.2001.04714.x
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发表时间：
2001-03
影响因子：
4.8
通讯作者：
P. Maxted;U. Heber;T. Marsh;R. S. U. O. Southampton;Keele University;Astronomisches Institut der Universitat Erlangen-Nurnberg-Astronomisch
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文献类型：
--
作者：
P. Maxted;U. Heber;T. Marsh;R. S. U. O. Southampton;Keele University;Astronomisches Institut der Universitat Erlangen-Nurnberg-Astronomisch
我们已经使用精确的径向速度测量的亚矮星-B星的帕洛玛-格林目录寻找二元极端水平分支（EHB）的明星。我们已经确定了有效的温度，表面重力和表面氦丰度的20个目标，从新的或现有的蓝色光谱，并编制了这些数量的出版值，但其他所有。我们确定了36 EHB星在我们的样本中，发现这些恒星中至少有21个是双星。除了一两个人之外，所有人都是新的身份。EHB星的最小双星比例为60± 8%，对轨道周期为P <$10 d的双星很敏感。对于双星的周期分布和质量比分布的合理假设，我们发现，在这个轨道周期范围内，我们的调查的平均探测效率为87%。短周期双星的比例（0.03 d <$P <$10 d）为69± 9%，该值与P <$10 d以下的周期分布或短周期双星的平均伴星质量无关。这些双星的轨道间距远小于形成EHB星星的红巨星的大小。这有力地证明了双星星星的演化是大多数EHB恒星形成的基础。如果也有轨道周期为10 d的EHB双星，那么受伴星影响的EHB星的比例可能会更高，例如，如果三分之一的EHB星是轨道周期为10 d <$P <$100 d的双星，那么我们的观测结果与所有的EHB星都是通过某种类型的星星演化形成的一致。我们发现，我们观察到的其他恒星中有五颗可能是后EHB恒星，其中一颗也是双星。
We have used precise radial velocity measurements of subdwarf-B stars from the Palomar–Green catalogue to look for binary extreme horizontal branch (EHB) stars. We have determined the effective temperature, surface gravity and surface helium abundance for 20 of the targets from new or existing blue spectra and have compiled published values for these quantities for all but one other. We identify 36 EHB stars in our sample and find that at least 21 of these stars are binaries. All but one or two of these are new identifications. The minimum binary fraction for EHB stars implied by our survey is 60±8 per cent. Our survey is sensitive to binaries with orbital periods P≲10 d. For reasonable assumptions concerning the period distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbital periods is 87 per cent. Allowing for this estimated detection efficiency, the fraction of EHB stars that are short-period binaries (0.03 d≲P≲10 d) is 69±9 per cent. The value is not strongly dependent on the period distribution below P≈10 d or the mean companion mass for these short-period binaries. The orbital separation of the stars in these binaries is much less than the size of the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars, the orbital periods of which are ≳10 d, the fraction of EHB stars for which evolution was affected by the presence of a companion may be much higher, e.g. if one third of EHB stars are binaries with orbital periods 10 d≲P≲100 d, then our observations are consistent with all EHB stars being formed through some type of binary star evolution. We find that five of the other stars we observed are likely to be post-EHB stars, one of which is also a binary.