arXiv 1102.16992011-02-08astro-ph.SR

在球状星团中的热水平分支恒星中缺乏近距离双星。II。NGC\，2808

A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC\,2808

C. Moni Bidin, S. Villanova, G. Piotto, Y. Momany

DOI：: 10.1051/0004-6361/201016232

摘要：

基于它们的双星起源的模型已经非常成功地再现了场亚矮星B的性质，但是在球状星团中观察它们的类似物提出了新的问题，而多个种群的发现为这些恒星的形成提供了一个有吸引力的替代方案。我们在NGC 2808中的蓝色水平分支恒星（Teff=12000-22000 K）样本中搜索周期P<200天的双星，该星团已知拥有三个不同的恒星星族和一个多模态水平分支。最终样本由64个目标组成。目标的径向速度在14个时期内测量，跨越约75天的时间间隔。在温度低于测光G1间隙的17000 K恒星中，我们没有发现RV变星，而在温度较高的目标中发现了两个近周期（P<10天）和两个中间周期（P=10-50天）系统。在95%的置信度下，温度低于间隙的恒星的近周期和中周期双星比例分别为fc<5%和fip<10%。在较热的恒星中，最可能的值是fc~20%和fip~ 30%，但90%的置信区间很大（分别为6-42%和11- 72%）。G1的差距出现作为一个不连续的二进制的派别，与较高的发病率的二进制之间的较热的恒星，但不断增加的f与温度，而不是不连续不能排除从我们的观察。我们发现，中期双星，从来没有调查过的集群HB恒星之间，可以发挥重要的作用，超过15-20%的最热的恒星，我们的样本。我们的研究结果表明，热HB星之间的FC是最有可能更高的年轻的集群，确认最近提出的年龄FC的关系。然而，观察到的星系团（如NGC 2808和NGC 6752）之间的二进制分数的巨大差异仍然没有再现的二进制人口合成模型。

英文摘要：

Models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars, but the observations of their analogues in globular clusters has posed new problems, while the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars. We search for binaries of period P<200 days among a sample of blue horizontal branch stars (Teff=12000-22000 K) in NGC2808, a cluster known to host three distinct stellar populations and a multimodal horizontal branch. The final sample consists of 64 targets. The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of about 75 days. We detect no RV variable object among stars cooler than the photometric G1 gap at 17000 K, while two close (P<10 days) and two intermediate-period (P=10-50 days) systems are found among hotter targets. The close and intermediate-period binary fraction for stars cooler than the gap are fc<5% and fip<10%, respectively, with 95% confidence. The most probable values among hotter stars are fc~20% and fip~30%, but the 90%- confidence level intervals are large (6-42% and 11-72%, respectively). The G1 gap appears as a discontinuity in the binary faction, with a higher incidence of binaries among hotter stars, but a constant increase in f with temperature rather than a discontinuity cannot be excluded from our observations. We find that intermediate-period binaries, never investigated before among cluster HB stars, could play an important role, being more than ~15-20% of the hottest stars of our sample. Our results indicate that fc among hot HB stars is most probably higher for younger clusters, confirming the recently proposed age-fc relation. However, the large observed difference in binary fraction between clusters (e.g. NGC2808 and NGC6752) is still not reproduced by binary population synthesis models.

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