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arXiv 0705.0894astro-ph
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早期R型星的起源：一个世纪问题的双星合并解决方案？

Origin of the early-type R stars: a binary-merger solution to a century-old problem?

Robert G. Izzard, C. Simon Jeffery, John Lattanzio

备注：
17 Pages, accepted for publication in A&A
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摘要：

早期的R型恒星是富碳的K型巨星。它们在C12、C13和N14中的含量都得到了增强，具有近似太阳的氧、镁同位素、s-过程和铁丰度，具有核心氦燃烧星的光度，不是快速旋转星，是银河系厚盘的成员，最特别的是，它们都是单星。传统的单星恒星演化模型无法解释这样的恒星，但双星系统的合并已经被提出来解释它们的起源。我们已经合成了双星群体，以计算合并的恒星的数量与氦核心，这可能是早期的R星。我们发现了许多可能的进化途径。其中最常见的是氦白矮星与燃烧氢的红巨星分支星在共同的包层阶段合并，然后在旋转的核心中氦闪光，将碳混合到表面。所有的通道加在一起给出的早期R星比我们最近的依巴谷观测所需的多十倍-我们讨论哪些通道可能是真正的早期R星，哪些不是。我们第一次构建了一个可行的早期R星模型，我们可以用它来测试我们的一些想法，关于巨星的共同信封演变，恒星合并，旋转，氦闪光和早期R星的起源。

英文摘要：

The early-R stars are carbon-rich K-type giants. They are enhanced in C12, C13 and N14, have approximately solar oxygen, magnesium isotopes, s-process and iron abundances, have the luminosity of core-helium burning stars, are not rapid rotators, are members of the Galactic thick disk and, most peculiarly of all, are all single stars. Conventional single-star stellar evolutionary models cannot explain such stars, but mergers in binary systems have been proposed to explain their origin. We have synthesized binary star populations to calculate the number of merged stars with helium cores which could be early-R stars. We find many possible evolutionary channels. The most common of which is the merger of a helium white dwarf with a hydrogen-burning red giant branch star during a common envelope phase followed by a helium flash in a rotating core which mixes carbon to the surface. All the channels together give ten times more early-R stars than we require to match recent Hipparcos observations - we discuss which channels are likely to be the true early-R stars and which are not. For the first time we have constructed a viable model of the early-R stars with which we can test some of our ideas regarding common envelope evolution in giants, stellar mergers, rotation, the helium flash and the origin of the early-R stars.

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