Yellow Post-asymptotic-giant-branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters
Yellow Post-asymptotic-giant-branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters
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DOI：
10.3847/1538-4357/ac674d
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发表时间：
2022-04
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影响因子：
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通讯作者：
R. Ciardullo;H. Bond;B. D. Davis;M. Siegel
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作者：
R. Ciardullo;H. Bond;B. D. Davis;M. Siegel
我们利用对银河系（和 LMC）球状星团中低表面重力恒星的调查结果来表明，“黄色”后渐近分支（yPAGB）恒星很可能是优秀的河外标准烛光，能够产生到早期型星系的距离，精确到几个百分点。我们发现球状星团中 10 颗 yPAGB 恒星的平均辐射星等为 <M bol> = −3.38 ± 0.03，该值比最新的后水平分支演化轨迹预测的值亮 ∼0.2 星等。更重要的是，我们表明观测到的分布色散仅为 0.10 星等，即优于单个造父变星的色散。我们描述了可以产生如此小的色散的物理原理，并表明，如果将勘测限制在颜色范围 0.0 ≲ (B − V)0 ≲ 0.5 内，那么可以很容易地以最小的污染来识别不变 yPAGB 恒星的样本。这些恒星极其明亮的绝对 V 星等 (<M V> = −3.37) 使它们成为迄今为止在古老恒星族群中视觉上最亮的天体，也是使用当前仪器测量到 ∼10 Mpc 的理想族群 II 标准烛光。因此，哈勃太空望远镜对 M81 和玉夫座星系晕的观测可以作为对造父变星和红巨星分支顶端距离尺度的有效交叉检查。
We use the results of a survey for low-surface-gravity stars in Galactic (and LMC) globular clusters to show that “yellow” post-asymptotic-branch (yPAGB) stars are likely to be excellent extragalactic standard candles, capable of producing distances to early-type galaxies that are accurate to a couple of percent. We show that the mean bolometric magnitude of the 10 yPAGB stars in globular clusters is 〈M bol〉 = −3.38 ± 0.03, a value that is ∼0.2 mag brighter than that predicted from the latest post-horizontal-branch evolutionary tracks. More importantly, we show that the observed dispersion in the distribution is only 0.10 mag, i.e., better than the scatter for individual Cepheids. We describe the physics that can produce such a small dispersion and show that, if one restricts surveys to the color range 0.0 ≲ (B − V)0 ≲ 0.5, then samples of nonvariable yPAGB stars can be identified quite easily with a minimum of contamination. The extremely bright absolute V magnitudes of these stars (〈M V 〉 = −3.37) make them, by far, the visually brightest objects in old stellar populations and ideal Population II standard candles for measurements out to ∼10 Mpc with current instrumentation. A Hubble Space Telescope survey in the halos of galaxies in the M81 and Sculptor groups could therefore serve as an effective cross-check on both the Cepheid and tip-of-the-red-giant-branch distance scales.