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arXiv 1003.2052astro-ph.GA
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半人马座欧米茄中蓝钩星的演化状态

The evolutionary status of the blue hook stars in Omega Centauri

Francesca D'Antona, Vittoria Caloi, Paolo Ventura

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摘要：

核心氦燃烧是极端水平分支恒星的主要能量来源，因为氢包层太小，无法提供核能输出。HR图中每个质量的演化都是沿着垂直轨道发生的，当核心氦被消耗时，演化到更高的Teff，然后到白矮星阶段。质量越大，模型的Teff越小，因此零年龄水平分支（ZAHB）是“水平的”。在本文中，我们表明，如果氦的质量分数（Y）的信封是大于Y~0.5，轨迹的形状完全改变：氢燃烧变得有效再次非常小的信封质量，由于较高的分子量和较高的温度的氢壳。Y越大，提供强H壳燃烧的包层质量越小。这些轨迹具有卷曲形状，位于遵循近似关系Teff=8090+ 32900 xY的Teff处，并且对于较大的包络质量变得更加明亮。因此，甚高氦模型的ZAHB在HR图中是“垂直的”。基于这些轨迹的合成模型很好地再现了球状星团wCen中“蓝钩”的位置和形状，最适合的是Y=0.80的非常高的Teff（更蓝）序列和Y=0.65的更冷（更红）序列。虽然Y的精确值可能取决于色-Teff转换，但我们知道，如果蓝钩星是星团蓝主序星的后代，那么它们的祖先星的氦含量不可能大于Y~0.38-0.40。因此，这种解释意味着所有这些天体实际上都是蓝主序星的后代，但它们都经历了进一步的深度混合，这在红巨星分支演化期间大大均匀地增加了它们的表面氦丰度。晚期氦闪光不可能是这种深度混合的原因，因为我们提出的模型具有比晚期闪光所需的更大质量的富氢包层。我们讨论了文献中提出的不同的深度混合模型，并得出结论，我们的解释蓝钩不能被排除，但需要更深入的调查才能被接受。

英文摘要：

Core helium burning is the dominant source of energy of extreme horizontal branch stars, as the hydrogen envelope is too small to contribute to the nuclear energy output. The evolution of each mass in the HR diagram occurs along vertical tracks that, when the core helium is consumed, evolve to higher Teff and then to the white dwarf stage. The larger is the mass, the smaller is the Teff of the models, so that the zero age horizontal branch (ZAHB) is "horizontal". In this paper we show that, if the helium mass fraction (Y) of the envelope is larger than Y~0.5, the shape of the tracks changes completely: the hydrogen burning becomes efficient again also for very small envelope masses, thanks to the higher molecular weight and to the higher temperatures of the hydrogen shell. The larger is Y, the smaller is the envelope mass that provides strong H-shell burning. These tracks have a curled shape, are located at a Teff following the approximate relation Teff=8090+ 32900xY, and become more luminous for larger envelope masses. Consequently, the ZAHB of the very high helium models is "vertical" in the HR diagram. Synthetic models based on these tracks nicely reproduce the location and shape of the "blue hook" in the globular cluster wCen, best fit by a very high Teff (bluer) sequence with Y=0.80 and a cooler (redder) one with Y=0.65. Although these precise values of Y may depend on the color-Teff conversions, we know that the helium content of the progenitors of the blue hook stars can not be larger than Y~0.38-0.40, if they are descendants of the cluster blue main sequence. Consequently, this interpretation implies that all these objects must in fact be progeny of the blue main sequence, but they have all suffered further deep mixing, that has largely and uniformly increased their surface helium abundance, during the red giant branch evolution. A late helium flash can not be the cause of this deep mixing, as the models we propose have hydrogen rich envelopes much more massive than those required for a late flash. We discuss different models of deep mixing proposed in the literature, and conclude that our interpretation of the blue hook can not be ruled out, but requires a much deeper investigation before it can be accepted.

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