The Globular Cluster NGC 5286. I. A New CCD BV Color-Magnitude Diagram

TL;DR

This study provides detailed BV photometry of globular cluster NGC 5286, deriving its metallicity, age, and horizontal branch morphology, and discusses its possible association with the Canis Major dwarf galaxy.

Contribution

First detailed BV color-magnitude diagram of NGC 5286 enabling precise age and metallicity analysis, and comparison with other clusters and galaxy components.

Findings

NGC 5286 has a metallicity of [Fe/H] ≈ -1.70 in Zinn & West scale.

NGC 5286 is approximately 1.7 Gyr older than M3.

NGC 5286's properties support its possible membership in the Canis Major dwarf galaxy.

Abstract

We present BV photometry of the Galactic globular cluster NGC 5286, based on 128 V frames and 133 B frames, and covering the entire face of the cluster. Our photometry reaches almost two magnitudes below the turn-off level, and is accordingly suitable for an age analysis. Field stars were removed statistically from the cluster's color-magnitude diagram (CMD), and a differential reddening correction applied, thus allowing a precise ridgeline to be calculated. Using the latter, a metallicity of [Fe/H] = -1.70 +/- 0.10 in the Zinn & West scale, and [Fe/H] = -1.47 +/- 0.02 in the Carretta & Gratton scale, was derived on the basis of several parameters measured from the red giant branch, in good agreement with the value provided in the Harris catalog. Comparing the NGC 5286 CMD with the latest photometry for M3 by P. B. Stetson (2008, priv. comm.), and using VandenBerg isochrones for a suitable chemical composition, we find evidence that NGC 5286 is around 1.7 +/- 0.9 Gyr older than M3. This goes in the right sense to help account for the blue horizontal branch of NGC 5286, for which we provide a measurement of several morphological indicators. If NGC 5286 is a bona fide member of the Canis Major dwarf spheroidal galaxy, as previously suggested, our results imply that the latter's oldest components may be at least as old as the oldest Milky Way globular clusters.