HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF WHITE-DWARFS IN THE GLOBULAR-CLUSTER M4

With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Tel escope, we have discovered in M4 (NGC 6121, C1620-264) the first exten sive sequence of cooling white dwarfs seen in a globular cluster. Adop ting a distance modulus of (m - M)(V) = 12.65 and a reddening of E(B - V) = 0.37, we show that the sequence, which extends over 9 < M(U) < 1 3, is comprised of white dwarfs of mass similar to 0.5 M,. The total m ass loss from the present turnoff to the white dwarf sequence is 0.31 M., and the intrinsic dispersion in the mean mass appears to be <0.05 M.. Both the location of the white dwarf cooling sequence in the clust er color-magnitude diagram and the cumulative luminosity function atte st to the basic correctness and completeness of the physics in theoret ical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond similar to 3 x 10(8) yr will require deeper and more compl ete data.