Zeroing the Stellar Isochrone Scale: The Red Giant Clump Luminosity at Intermediate Metallicity
Zeroing the Stellar Isochrone Scale: The Red Giant Clump Luminosity at Intermediate Metallicity
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DOI：
10.1086/300810
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发表时间：
1999-01
期刊：
影响因子：
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通讯作者：
B. Twarog;B. Anthony-Twarog;Andrew R. Bricker
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文献类型：
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作者：
B. Twarog;B. Anthony-Twarog;Andrew R. Bricker
疏散星团NGC 2420和NGC 2506的色星等图是太阳邻域和麦哲伦云之间的中间联系。两套理论上的等时线（包括对流过冲）在太阳丰度处归零到太阳，在[Fe/H] =-0.4处归零到依巴谷星表中未演化的主序星矮星，在给定的颜色下，未演化的主序星之间需要0.4等的差异。对NGC 2506和NGC 2420分别取E（B-V）= 0.04和[Fe/H] = -0.39和E（B-V）= 0.04和[Fe/H] = -0.29，其表观模量分别为（m-M）= 12.70和12.15，年龄分别为1.9 ± 0.2 Gyr和2.2 ± 0.2 Gyr，这取决于等时线的选择。在两个星团的合成巨星分支中，V和I的平均星团星等分别为+0.47和-0.48（-0.17，+0.14）。对MI值进行金属丰度校正后，Udalski的聚类样本导致LMC和SMC的（m-M）0分别为18.42（+0.17，-0.15）和18.91（+0.18，-0.16）。对这一讨论的一个警告，以及可能对相同丰度和年龄的星系团是相同的说法的警告是，观察到NGC 2506中红巨星的V-I颜色在给定的B-V下比年龄和/或金属量相当的星系团中的巨星明显更红。上述距离标度是使用B-V和V-I之间的一般聚类关系导出的。如果NGC 2506中的CCD测光与标准系统正确连接，则团块的MI将减小，距离模量应增加0.1 mag。
The color-magnitude diagrams of the open clusters NGC 2420 and NGC 2506 have been investigated as intermediate links between the solar neighborhood and the Magellanic Clouds. Two sets of theoretical isochrones that include convective overshoot are zeroed to the Sun at solar abundance and to the unevolved main-sequence dwarfs of the Hipparcos Catalogue at [Fe/H] = -0.4, requiring a differential of 0.4 mag between the unevolved main sequences at a given color. Adopting E(B-V) = 0.04 and [Fe/H] = -0.39 for NGC 2506 and E(B-V) = 0.04 and [Fe/H] = -0.29 for NGC 2420, the respective apparent moduli are (m-M) = 12.70 and 12.15, while the ages of both clusters are approximately 1.9 ± 0.2 Gyr or 2.2 ± 0.2 Gyr, depending on the choice of isochrones. From the composite giant branch of the two clusters, the mean clump magnitudes in V and I are found to be +0.47 and -0.48 (-0.17, +0.14), respectively. Applying a metallicity correction to the MI values, the cluster sample of Udalski leads to (m-M)0 = 18.42 (+0.17, -0.15) and 18.91 (+0.18, -0.16) for the LMC and SMC, respectively. A caveat to this discussion and potentially to the claim that clusters of the same abundance and age are identical is the observation that the V-I colors of the red giants in NGC 2506 are significantly redder at a given B-V than the giants in clusters of comparable age and/or metallicity. The distance scale above has been derived using the general cluster relation between B-V and V-I. If the CCD photometry in NGC 2506 is correctly tied to the standard system, MI for the clump will decrease and the distance moduli should increase by 0.1 mag.