The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)

Abstract

We present deep CCD photometry of the very rich, intermediate-age (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well-populated and very tightly constrained main sequence extending over 12 mag from the turnoff. Both a well-defined main-sequence turnoff and a red giant population are also clearly evident. The CFH12K photometry for this cluster is faint enough (V ∼ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf "clump" caused by the decreased rate of cooling of these stars as they age, and a subsequent gap with very few objects. By using source classification to eliminate faint blue galaxies and a statistical subtraction technique to eliminate foreground/background objects, we have determined the age of the star cluster from the termination point (MV = 11.95 ± 0.30) in the white dwarf luminosity function. The white dwarf cooling age is found to be 566 ± Myr from comparisons with white dwarf cooling models and is in excellent agreement with the main-sequence turnoff isochrone age (520 Myr). After carefully accounting for the uncertainties in the white dwarf limiting magnitude, we show that the cooling age confirms that models including convective core overshooting are preferred for young–intermediate-aged clusters. This is particularly important in the case of NGC 2099 as the age is similar to that of the Hyades cluster, for which current models have difficulties in reproducing the details of the main-sequence turnoff. We also derive the reddening [E(B-V) = 0.21 ± 0.03] and distance [(m - M)V = 11.55 ± 0.13] to NGC 2099 by matching main-sequence features in the cluster to a new fiducial main sequence for the Hyades, after correcting for small metallicity differences. As a continuing part of the goals of the CFHT Open Star Cluster Survey to better understand dynamical processes of open clusters, we also fit a King model to the cluster density distribution and investigate the cluster main-sequence luminosity and mass functions in increasing concentric annuli. We find some evidence for mass segregation within the boundary of NGC 2099 as expected given the cluster's age relative to the dynamical age. The present global mass function for the cluster is found to be shallower than a Salpeter IMF.