The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

Abstract

We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC 2323 contains approximately 2100 stars brighter than our photometric limit of V = 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 magnitudes in the V, B-V plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc). Analysis of the luminosity and mass functions suggest that despite their young ages, both clusters are somewhat dynamically relaxed exhibiting signs of mass-segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates which may be the remnants of quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turn-off age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and upper mass limit for white dwarf production.