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 _ { \odot } - about 0.2 M _ { \odot } larger than the mean seen among 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 \sim 3.5 .
While the probability of seeing no DB WDs in NGC 2099 solely by chance is \sim 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 .