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arXiv 1009.3191astro-ph.SR
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半人马座ω中的热水平分支恒星

The hot horizontal-branch stars in omega Centauri

S. Moehler, S. Dreizler, T. Lanz, G. Bono, A. V. Sweigart, A. Calamida, M. Nonino

备注：
11 pages, 8 figures, uses aa.cls (included), A&A in press
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摘要：

对一些大质量球状星团的紫外线观测揭示了大量比水平分支（HB）的热端更热和更暗的恒星，即所谓的蓝钩星。这一特征可以用热闪晚期的情景来解释，在这种情景中，恒星在白矮星冷却曲线上经历氦闪，或者用一些星团中假设存在的富氦子种群的后代来解释。之前对欧米茄Cen和NGC 2808中蓝钩星的光谱分析支持后期热闪光的假设，但恒星中的氦含量比预期的要少得多，预测的C和N富集度无法得到验证。我们比较了观测到的有效温度，表面重力，氦丰度，碳线强度（可检测）我们的目标恒星与上述两种情况的预测。使用LTE和非LTE模型大气，对球状星团omega Cen中热HB星的中高分辨率光谱进行了径向速度变化、大气参数和丰度分析。我们在目标恒星中没有发现近距离双星的证据。所有低于30，000 K的恒星都是贫氦的，与其他球状星团中在该温度范围内观测到的HB星非常相似。在30，000 K到50，000 K的温度范围内，我们发现28%的恒星是贫氦的（log（He/H）< -1.6），而72%的恒星具有近似太阳或超太阳的氦丰度（log（He/H）>= -1.5）。我们还发现，碳富集与氦富集密切相关，最大碳富集质量为3%。一个强大的碳富集与氦富集串联预测的后期热闪光的情况下，但不是由氦富集的情况。我们的结论是，富含氦的HB星在欧米茄Cen不能被解释的氦富集的情况下调用来解释蓝色的主序列。

英文摘要：

UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios. Moderately high resolution spectra of hot HB stars in the globular cluster omega Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. We find no evidence of close binaries among our target stars. All stars below 30,000K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30,000K to 50,000K, we find that 28% of our stars are helium-poor (log(He/H) < -1.6), while 72% have roughly solar or super-solar helium abundance (log(He/H) >= -1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.

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