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

We present deep CCD photometry of the very rich, intermediate aged (similarto the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitudediagram (CMD) for the cluster shows an extremely well populated and verytightly constrained main-sequence extending over 12 magnitudes from theturn-off. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) todetect the remnants of the most massive progenitor cluster stars under the TypeI SNe limit. Therefore, the CMD of the cluster also exhibits a well definedwhite dwarf `clump' caused by the decreased rate of cooling of these stars asthey age, and a subsequent gap with very few objects. The termination point (Mv= 11.95 +/- 0.30) of the white dwarf luminosity function gives a white dwarfcooling age of 566 +/- 154/176 Myrs which is in excellent agreement with themain-sequence turn-off isochrone age (520 Myrs). By carefully accounting forerrors, we show that the cooling age confirms that models including convectivecore overshooting are preferred for young-intermediate aged clusters. We alsoderive 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 newfiducial main-sequence for the Hyades. As a continuing part of the goals of theCFHT Open Star Cluster Survey to better understand dynamical processes of openclusters, we also fit a King model to the cluster density distribution andinvestigate the cluster main-sequence luminosity and mass functions inincreasing concentric annuli. We find some evidence for mass segregation withinthe boundary of NGC 2099 as expected given the cluster's age relative to thedynamical age. The present global mass function for the cluster is found to beshallower than a Salpeter IMF.