标题： 氦分布在NGC 2808的主要序列星中 显示英文标题

标题： A helium spread among the main sequence stars in NGC 2808

摘要： 我们研究了球状星团 NGC 2808 的主序星颜色分布，基于对该星团外围未拥挤区域一个场的新深空 HST-WFPC2 光度测量。考虑到我们仔细确定的测量误差，主序星的颜色分布比单一恒星群体预期的更宽。大约 20% 的样本恒星比预期要蓝得多，最可能被解释为具有比主序星大部分更高的氦丰度的一个群体。我们估计这些恒星的氦质量分数约为 Y ~ 0.4。NGC 2808 可能经历了自我富集过程，不同的恒星群体诞生于第一代中间质量渐近巨星分支（AGB）恒星喷发的残骸中。除了 Y=0.40 的恒星外，约 30% 的恒星应具有分布在 Y=0.26-0.29 范围内的氦丰度，而 50% 的恒星具有原始氦丰度，以解释水平分支形态的特殊性。识别出三个主要的恒星形成阶段：第一个具有原始氦含量 Y ~ 0.24；第二个由第一代最巨大 AGB 恒星（6-7 倍太阳质量）的风形成，具有 Y ~ 0.4；第三个由较轻 AGB 恒星（~ 3.5-4.5 倍太阳质量）喷发的物质形成，具有 Y ~ 0.26-0.29。在第二代和第三代之间存在长时间间隔（数千万年），晚期超新星 II 的爆炸可能由于双星演化抑制了恒星形成。 显示英文摘要

摘要： We studied the color distribution of the main sequence of the Globular Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The color distribution of main sequence stars is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. We estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may have suffered self-enrichment, with different stellar populations born from the ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the first generation. In addition to the Y=0.40 stars, roughly 30% of the stars should have Y distributed between 0.26-0.29 while 50% have primordial Y, to explain also the peculiar horizontal branch morphology. Three main stages of star formation are identified, the first with primordial helium content Y ~ 0.24, the second one born from the winds of the most massive AGBs of the first stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a long hiatus of time (several 10^7yr) between the second and third generation, star formation might have been inhibited by the explosion of late Supernovae II deriving from binary evolution.