The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters

Spectra have been obtained of 21 white dwarfs (WDs) in the direction of the young, rich open star cluster NGC 2099. This represents an appreciable fraction (> 30%) of the cluster’s total WD population. The mean derived mass of the sample is 0.8 M⊙ about 0.2 M⊙ larger than the mean seen amongst field WDs. 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. The number ratio in the field at the temperatures of the NGC 2099 WDs is DA/DB ∼ 3.5. While the probability of seeing no DB WDs in NGC 2099 solely by chance is ∼ 2%, if we include WDs in other open clusters of similar age it then becomes highly unlikely that the dearth of DB WDs in young open clusters is just a statistical fluctuation. We explore possible reasons for the lack of DBs in these clusters including the accretion of hydrogen-rich cluster gas by the WDs. We conclude that this is unlikely to be the cause as the cluster accretion rate is probably similar to that for field stars. Masses of field DB WDs are only poorly constrained but they appear to be largely confined below 0.8M⊙. From a sequence of cooling models of various masses it appears that the most promising scenario for the DA/DB number ratio discrepancy in young clusters 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. Subject headings: open clusters: individual (NGC 2099) – stars: white dwarfs