Studying white dwarfs in globular clusters provides some unique advantages over studying them in the field. A globular cluster presents us with a coeval population of known age, distance and metal abundance. The white dwarfs have then evolved from a well characterised precursor population. In this contribution, I examine three aspects of white dwarfs, all of which use the unique properties of a globular cluster. The cluster colour-magnitude diagram (CMD) allows for a study of the white dwarf cooling sequence. I present such a diagram forNGC 6397 which we recently observed with the Hubble Space Telescope (HST). This very deep integration has allowed us to reach a truncation in the cooling sequence. This will allow for the determination of a precise white dwarf cooling age for the cluster which rivals in precision the WMAP age of the Universe (Hansen et al. 2007). In addition the white dwarfs we are observing at the faint (cool) end of the cooling sequence are cool enough that the effect of collision-induced absorption (CIA) in hydrogen is observable in the CMD. CIA makes its presence known by forcing the coolest white dwarfs to have bluer colours as they cool (Richer et al. 2006). We have obtained spectra with Gemini South of some of the brighter white dwarfs in Messier 4. While the spectra have only modest signal-to-noise ratios, they are more than adequate to investigate the spectral types of these bright white dwarfs. Following in step with what we found in open clusters (Kalirai et al. 2005), all of the 15 brightest Messier 4 white dwarfs possess DA spectra (Davis et al. 2007a) whereas wewould have expected 3-4 to be DBs. There is no current obvious explanation for why a cluster environment preferentially produces hydrogen-rich atmosphere white dwarfs. Lastly, I explore the radial distributions of white dwarfs in both the globular clusters Messier 4 and NGC 6397. The clusters, both of which were observed with HST, seem to indicate that young white dwarfs (those with cooling ages < 1 Gyr) have a more extended radial distribution than old white dwarfs (ages > 1.5 Gyr) (Davis et al. 2007b). The simplest explanation for this result is that white dwarfs are born with a modest kick (sim3 km/s) and eventually relax to have a distribution characteristic of their mass.
15th European Workshop on White Dwarfs
- Pub Date:
- September 2007