On the radial distribution of white dwarfs in the globular cluster NGC 6397

We have examined the radial distribution of white dwarfs over a single Hubble Space Telescope/Advanced Camera for Surveys field in the nearby globular cluster NGC 6397. In relaxed populations, such as in a globular cluster, stellar velocity dispersion, and hence radial distribution, is directly dependent on stellar masses. The progenitors of very young cluster white dwarfs had a mass of ~0.8M_solar, while the white dwarfs themselves have a mass of ~0.5M_solar. We thus expect young white dwarfs to have a concentrated radial distribution (like that of their progenitors) that becomes more extended over several relaxation times to mimic that of ~0.5-M_solar main-sequence stars. However, we observe young white dwarfs to have a significantly extended radial distribution compared both with the most massive main-sequence stars in the cluster and with old white dwarfs.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with proposals GO-10424 and GO-8679.

E-mail: sdavis@astro.ubc.ca (DSD); richer@astro.ubc.ca (HBR) ‡

Hubble Fellow.