The initial-final mass relationship: Spectroscopy of white dwarfs in NGC 2099 (M37)

We present new observations of very faint white dwarfs (WDs) in the rich open star cluster NGC 2099 (M37). Following deep, wide-field imaging of the cluster using the Canada-France-Hawaii Telescope, we have now obtained spectroscopic observations of candidate WDs using both the Gemini Multi-Object Spectrograph on Gemini North and the Low-Resolution Imaging Spectrometer on Keck. Of our 24 WD candidates ( all fainter than V = 22.4), 21 are spectroscopically confirmed to be bona fide WDs, four or five of which are most likely V p 22.4 field objects. Fitting 18 of the 21 WD spectra with model atmospheres, we find that most WDs in this cluster are quite massive (0.7-0.9 M-circle dot), as expected given the cluster's young age (650 Myr) and, hence, high turnoff mass (similar to2.4 M-circle dot). We determine a new initial-final mass relationship and almost double the number of existing M, data points from previous studies. The results indicate that stars with initial masses between 2.8 and 3.4 M-circle dot lose 70%-75% of their mass through stellar evolution. For the first time, we find some evidence of a metallicity dependence on the initial-final mass relationship.