vby C a Hβ CCD Photometry of Clusters. VI. The Metal-deficient Open Cluster NGC 2420

Abstract

CCD photometry on the intermediate-band vbyCaHβ system is presented for the metal-deficient open cluster NGC 2420. Restricting the data to probable single members of the cluster using the CMD and the photometric indices alone generates a sample of 106 stars at the cluster turnoff. The average E(b-y)=0.035+/-0.003 (s.e.m.) or E(B-V)=0.050+/-0.004 (s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m1 and hk, using b-y and Hβ as the temperature index. The agreement among the four approaches is reasonable, leading to a final weighted average of [Fe/H]=-0.37+/-0.05 (s.e.m.) for the cluster, on a scale where the Hyades has [Fe/H]=+0.12. When combined with the abundances from DDO photometry and from recalibrated low-resolution spectroscopy, the mean metallicity becomes [Fe/H]=-0.32+/-0.03. It is also demonstrated that the average cluster abundances based on either DDO data or low-resolution spectroscopy are consistently reliable to 0.05 dex or better, contrary to published attempts to establish an open cluster metallicity scale using simplistic offset corrections among different surveys.