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The second and third parameters of the horizontal branch in globular clusters

机译：球状星团水平分支的第二个和第三个参数

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Context. The second parameter (the first being metallicity)defining the distribution of stars on the horizontal branch (HB) ofglobular clusters (GCs) has long been one of the major open issues inour understanding of the evolution of normal stars. Large photometricand spectroscopic databases are now available: they include large andhomogeneous sets of colour-magnitude diagrams, cluster ages, andhomogeneous data about chemical compositions from our FLAMES survey. Aims. We use these databases to re-examine this issue. Methods. We use the photometric data to derive median andextreme (i.e., the values including 90% of the distribution) coloursand magnitudes of stars along the HB for about a hundred GCs. Wetransform these into median and extreme masses of stars on the HB,using the models developed by the Pisa group, and taking into accountevolutionary effects. We compare these masses with those expected atthe tip of the red giant branch (RGB) to derive the total mass lost bythe stars. Results. We find that a simple linear dependence on metallicityof this total mass lost describes quite well the median colours ofHBstars. Assuming this mass loss law to be universal, we findthat age is the main second parameter, determining many of the mostrelevant features related to HBs. Inparticular, itallows usto explain the Oosterhoff dichotomy as a consequence of the peculiarage-metallicity distribution of GCs in our Galaxy, although bothOosterhoff groups have GCs spanning a rather wide range of ages.However, at least an additional- third- parameter isclearly required. The most likely candidate is the He abundance, whichmight be different in GCstars belonging to the different stellargenerations whose presence was previously derived from the Na-O andMg-Al anticorrelations. Variations in the median He abundance allow usto explain the extremely blue HB of GCs like NGC6254 (=M10)and NGC1904 (=M79); such variations are found to be(weakly) correlated with the values of the R-parameter (that isthe ratio of the number of stars on the HB and on the RGB). We alsoshow that suitable Heabundances allow deriving ages from the HBwhich are consistent with those obtained from the Main Sequence. Smallcorrections to these latter ages are then proposed. We find that a verytight age-metallicity relation (with a scatter below4%) can beobtained for GCs kinematically related to the disk and bulge, oncethese corrections are applied. Furthermore, star-to-star variations inthe Hecontent, combined with a small random term, explain verywell the extension of the HB. There is a strong correlation betweenthis extension and the interquartile of the Na-Oanticorrelation,strongly supporting the hypothesis that the third parameter for GC HBsis He. Finally, there are strong indications that the main driver forthese variations in the He-content within GCs is the total clustermass. There are a few GCs exhibiting exceptional behaviours (includingNGC104=47Tuc and in less measure NGC5272=M3);however, they can be perhaps accommodated in a scenario for theformation of GCs that relates their origin to cooling flows generatedafter very large episodes of star formation, as proposed by Carrettaetal. (2009d). Key words: globular clusters: general - stars: abundances
机译：上下文。定义恒星在球状星团（GC）的水平分支（HB）上的分布的第二个参数（第一个是金属性）一直是我们了解正常恒星演化的主要开放问题之一。现在可以使用大型的光度学和光谱数据库：它们包括大型且同质的色度图，聚类年龄以及我们FLAMES调查中有关化学成分的同质数据。目的我们使用这些数据库来重新检查此问题。方法。我们使用光度学数据推导了大约100个GC沿HB的恒星的中值和极值（即包括90％分布的值）的颜色和大小。我们使用比萨小组开发的模型，并考虑到演化效应，将其转换为HB上恒星的中值和极值质量。我们将这些质量与红色巨型分支（RGB）末端的预期质量进行比较，以得出恒星损失的总质量。结果。我们发现，此总质量损失对金属性的简单线性依赖性很好地描述了HBstars的中位颜色。假定该质量损失定律是普遍适用的，我们发现年龄是主要的第二个参数，它确定了许多与HBs相关的特征。特别是，尽管两个Oosterhoff组的GC年龄跨度都相当大，但我们还是可以解释Oosterhoff二分法是由于GC在我们银河系中的特殊性-金属性分布所致。但是，显然至少需要附加的第三个参数。最可能的候选者是He丰度，He丰度在属于不同恒星世代的GCstar中可能有所不同，这些恒星的存在先前源自Na-O和Mg-Al反相关。中位数氦气丰度的变化使我们能够解释诸如NGC6254（= M10）和NGC1904（= M79）之类的GC的极蓝HB。发现这种变化与R参数的值（即HB和RGB上的恒星数之比）（弱）相关。我们还表明，适当的丰度允许从HB推导年龄，该年龄与从主序列获得的年龄一致。然后提出对这些较晚年龄的小修正。我们发现，一旦应用了这些校正，就可以在运动学上与圆盘和凸出相关的GC上获得非常紧密的年龄-金属关系（散射低于4％）。此外，Hecontent的逐星变化，再加上一个小的随机术语，很好地解释了HB的扩展。此扩展与Na-Oanticorrelation的四分位数之间有很强的相关性，有力地支持了GC HBsis He的第三个参数的假设。最后，有强有力的迹象表明，GC中He含量的主要驱动因素是总簇质量。有一些GC表现出异常的行为（包括NGC104 = 47Tuc，在较小程度上为NGC5272 = M3）;但是，也许可以将它们容纳在形成GC的场景中，该场景将其起源与很大的恒星形成后产生的冷却流联系起来，如Carrettaetal所建议。 （2009d）。关键词：球状星团：一般-星星：丰度

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