White Dwarfs in Globular Clusters:Hubble Space TelescopeObservations of M4

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radialdistances from the cluster center were observed and sizeable WD populationswere found in all three. The location of these WDs in the color-magnitudediagram, their mean mass of 0.51($ \pm 0.03$)M$_{\odot}$, and their luminosityfunction confirm basic tenets of stellar evolution theory and support theresults from current WD cooling theory. The WDs are used to extend the clustermain-sequence mass function upward to stars that have already completed theirnuclear evolution. The WD/red dwarf binary frequency in M4 is investigated andfound to be at most a few percent of all the main-sequence stars. The mostancient WDs found are about 9 Gyr old, a level which is set solely by thephotometric limits of our data. Even though this is less than the age of M4, wediscuss how these cooling WDs can eventually be used to check the turnoff agesof globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the Astrophysical Journa