The Initial-Final Mass Relation and the DA/DB White Dwarf Ratio

We present spectroscopic observations of very faint white dwarfs in the rich open star cluster NGC 2099 (M37). With multiobject data from both GMOS on Gemini and LRIS on Keck, we confirm the true WD nature for 21 of 24 faint WD candidates (V > 22.4), all of which were previously identified as possible WDs through CFHT imaging. Fitting 18 of the 21 WD spectra with model atmospheres, we find that the mean derived mass of the sample is 0.8 Msun - about 0.2 Msun larger than the mean seen amongst field WDs. This is expected given the clusters young age (650 Myrs), and hence, high turn-off mass ( ∼ 2.4 M_⊙). A surprising result is that all of the NGC 2099 WDs have hydrogen-rich atmospheres (DAs) and none exhibit helium-rich ones (DBs), or any other spectral class. From a sequence of cooling models of various masses it appears that the most promising scenario for the DA/DB number ratio discrepancy is that hot, high mass WDs do not develop large enough helium convection zones to allow helium to be brought to the surface and turn a hydrogen-rich WD into a helium-rich one. We also determine a new initial-final mass relationship and nearly double the number of existing data points from previous studies. The results indicate that stars with initial masses between 2.8 and 3.4 Msun lose 75% of their mass through stellar evolution.

We wish to thank the Gemini, Keck and Canada-France-Hawaii Telescopes. J.S.K. acknowledges support from an NSERC PGS-B Graduate Fellowship.