The dearth of massive, helium-rich white dwarfs in young open star clusters.
KALIRAI J.S., RICHER H.B., HANSEN B.M.S., REITZEL D. and RICH R.M.
Abstract (from CDS):
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 among field WDs. A surprising result is that all of the NGC 2099 WDs have hydrogen-rich atmospheres (DAs); 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 and conclude 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.
Galaxy: Open Clusters and Associations: Individual: NGC Number: NGC 2099 - Stars: White Dwarfs