Zeroing the Stellar Isochrone Scale: The Red Giant Clump Luminosity at Intermediate Metallicity

Abstract

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.