White Dwarfs in Globular Clusters:<i>Hubble Space Telescope</i>Observations of M4

Abstract

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258\nwhite dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial\ndistances from the cluster center were observed and sizeable WD populations\nwere found in all three. The location of these WDs in the color-magnitude\ndiagram, their mean mass of 0.51($ \\pm 0.03$)M$_{\\odot}$, and their luminosity\nfunction confirm basic tenets of stellar evolution theory and support the\nresults from current WD cooling theory. The WDs are used to extend the cluster\nmain-sequence mass function upward to stars that have already completed their\nnuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and\nfound to be at most a few percent of all the main-sequence stars. The most\nancient WDs found are about 9 Gyr old, a level which is set solely by the\nphotometric limits of our data. Even though this is less than the age of M4, we\ndiscuss how these cooling WDs can eventually be used to check the turnoff ages\nof globular clusters and hence constrain the age of the Universe.\n