On a New Parameter to Estimate the Helium Content in Old Stellar Systems
On a New Parameter to Estimate the Helium Content in Old Stellar Systems
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DOI：
10.1086/661534
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发表时间：
2011-06
影响因子：
3.5
通讯作者：
F. Troisi;G. Bono;P. Stetson;A. Pietrinferni;A. Weiss;M. Fabrizio;I. Ferraro;A. D. Cecco;G. Iannicola;R. Buonanno;A. Calamida;F. Caputo;C. Corsi;M. Dall’ora;A. Kunder;M. Monelli;M. Nonino;A. Piersimoni;L. Pulone;M. Romaniello;A. Walker;M. Zoccali
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作者：
F. Troisi;G. Bono;P. Stetson;A. Pietrinferni;A. Weiss;M. Fabrizio;I. Ferraro;A. D. Cecco;G. Iannicola;R. Buonanno;A. Calamida;F. Caputo;C. Corsi;M. Dall’ora;A. Kunder;M. Monelli;M. Nonino;A. Piersimoni;L. Pulone;M. Romaniello;A. Walker;M. Zoccali
我们引入了一个新的参数Δ θ-红巨星分支（RGB）凸起和主序（MS）上与凸起相同颜色的点（我们称之为“基准”）之间的星等差异-来估计旧恒星系统中的氦含量。Δ λ参数是一个氦指标，因为在固定的年龄和铁丰度下，氦的增加会使RGB凸起变亮，而MS基准变暗。此外，它对氦的灵敏度在整个金属丰度范围内是线性的。星系团年龄的几个千兆年的变化、光度零点的不确定性、红化的数量，或者水平分支的演化效应，也对Δ λ的影响很小。Δ Y参数的两个主要缺点包括需要精确和大型的光度数据集，以及ΔY/Δ Y斜率对金属丰度的强烈依赖。我们选择了近24个相对明亮的银河系球状星团（GGC），它们具有低前景红化和广泛的铁丰度范围（-2.45 ≤ [Fe/H] ≤ -0.70 dex），以及精确和均匀的多波段（BVI）测光。如果我们假设原始氦的含量Y = 0.20（质量丰度），那么观测到的Δ ε参数和α增强演化模型预测的Δ ε参数相当吻合。只有在贫金属区，这种差异才变为Δ 4σ（ΔB = 0.20 mag）。与基于规范原始氦含量（Y = 0.245）的处方进行比较表明，观察到的Δ ε值小于预测值。这些发现支持了先前的结果，表明当前的进化模型高估了RGB凸起的亮度。包络超调的加入可以消除这种差异。GGC的Δ δ参数提供了星前氦的独立探测，至少在5σ水平上。
We introduce a new parameter Δξ—the difference in magnitude between the red giant branch (RGB) bump and the point on the main sequence (MS) at the same color as the bump (which we call the “benchmark”)—to estimate the helium content in old stellar systems. The Δξ parameter is a helium indicator, since an increase in helium makes, at fixed age and iron abundance, the RGB bump brighter and the MS benchmark fainter. Moreover, its sensitivity to helium is linear over the entire metallicity range. Δξ is also minimally affected by changes of a few gigayears in cluster age, by uncertainties in the photometric zero point, by the amount of reddening, or by the effects of evolution on the horizontal branch. The two main drawbacks of the Δξ parameter include the need for precise and large photometric data sets and a strong dependence of the ΔY/Δξ slope on metallicity. We selected almost two dozen relatively bright Galactic globular clusters (GGCs) with low foreground reddening and a broad range of iron abundance (-2.45 ≤ [Fe/H] ≤ -0.70 dex), together with precise and homogeneous multiband (BVI) photometry. The observed Δξ parameters and those predicted from α-enhanced evolutionary models agree reasonably well if we assume a primordial helium content of Y = 0.20 (abundance by mass). The discrepancy becomes ∼4σ (ΔB = 0.20 mag) only in the metal-poor regime. Comparison with prescriptions based on a canonical primordial helium content (Y = 0.245 indicates that the observed Δξ values are smaller than predicted. These findings support previous results suggesting that current evolutionary models overestimate the luminosity of the RGB bump. The inclusion of envelope overshooting can eliminate the discrepancy. The Δξ parameter of GGCs provides an independent detection of prestellar helium, at least at the 5σ level.