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

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

Jan 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005apj...618l.129k&link_type=abstract

The Astrophysical Journal, Volume 618, Issue 2, pp. L129-L132.

Astronomy and Astrophysics

Astronomy

18

Galaxy: Open Clusters And Associations: Individual: Ngc Number: Ngc 2099, Stars: White Dwarfs

Scientific paper

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 Msolar-about 0.2 Msolar 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.
Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck. Gemini is an international partnership managed by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. The W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, was made possible by the generous financial support of the W. M. Keck Foundation.

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