The Hydrogen-Deficient Carbon Stars
The Hydrogen-Deficient Carbon Stars
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缺氢碳星
DOI：
10.1093/mnras/137.2.119
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发表时间：
1967
影响因子：
4.8
通讯作者：
B. Warner
中科院分区：
文献类型：
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作者：
B. Warner
对五颗已知的具有弱氢线的不变碳星进行了概略分析。可能除了Na之外，所有比O重的可观测元素相对于Fe都有正常的丰度。其中4颗星的H值不足105倍以上，其余星星的H值不足50倍，C值过剩3 ~ 10倍。强调了上述恒星、R Cr B变星和氦星在成分、空间和速度分布以及绝对星等方面的相似性。这些恒星被统称为缺氢碳（HDC）星和他们的各种性质进行了讨论。它们似乎形成了一类与普通R星和N星完全不同的星星。冷Hd C星中的连续不透明度被证明是由He-、C-、电子散射、He和C的瑞利散射以及碳的光电离所引起的。结果表明，每克恒星物质中的Fe原子数是太阳中的2 ~ 0.3倍。大多数迹象表明，Hd-C星的金属元素略有不足，这与它们的大空间速度相一致，我们认为，这些天体中H、C、N和O的异常丰度是由于在一颗1 MO的星星中He燃烧开始后不久，外层包层的损失引起的。预测的丰度与观测结果基本一致，表明大部分O将以O 18的形式存在。Hd-C阶段的寿命似乎为10103年，在这种情况下，长期变化应该是可以观测到的。提出的演化方案显示了这些氦星是如何以红超巨星的形式出现并演化成HR图中的白色矮星区域的。
Abundance analyses of the five known non-variable carbon stars with weak hydrogen lines are carried out. With the possible exception of Na all observable elements heavier than O have normal abundances relative to Fe. H is deficient in four of the stars by factors of more than 105; in the remaining star a deficiency of a factor of 50 is found. C is over-abundant by factors of 3 to 10.The similarity of the compositions, space and velocity distributions and absolute magnitudes of the above stars, the R Cr B variables and the helium stars is stressed. These stars are collectively called hydrogen-deficient carbon (Hd C) stars and their various properties are discussed. They appear to form a class of star entirely different from the normal R and N stars.The continuous opacity in the cool Hd C stars is shown to arise from contributions from He−, C−, electron scattering, Rayleigh scattering by He and C, and photoionization of carbon. It is shown that the number of Fe atoms per gram of stellar material lies in the range 2 to 0.3 times that in the Sun. Most indications are that the Hd C stars are slightly metal deficient, in agreement with their large space velocities.It is suggested that the peculiar abundances of H, C, N and O in these objects arise from loss of outer envelope at a stage shortly after He-burning has started in a star of 1MO. The predicted abundances are then in reasonable agreement with observations, and it emerges that most of the O will be in the form of O18.The lifetime of the Hd C stage appears to be ∼ 103years, in which case secular changes should be observable. The proposed scheme of evolution shows how these helium stars appear as red supergiants and evolve into the white dwarf region of the HR diagram.