Ultraviolet-Excess Objects in the Field of NGC 6121 (M4)

The statistics of a blink survey designed to select ultraviolet-excess objects are presented for a 50-arcmin diameter field centered on the globular cluster M4. The search area between 3.9 arcmin and 23.7 arcmin from the cluster center was found to contain 1096 such objects brighter than U = 22.3 mag. These objects exhibited a radial concentration toward the center of the cluster. Interpreting this excess of faint blue objects as an indication of the presence of a hot white dwarf component in M4, subsequent subtraction of the estimated background unveiled a radial gradient similar to that of King, Hedemann, and Hodge's star counts in the same region. About 240 white dwarfs are estimated to be present in the area searched brighter than M(v) = about 11. UBV photometry of 74 of the objects is also presented, showing that at least six objects have colors and photometric parallaxes consistent with those expected for very hot cluster white dwarfs. Association with the cluster would imply that these six have M(v) less than 9.4. Theoretical estimates from Fusi-Pecci and Renzini and from Hills are compared with the observations. The predictions do not appear to be inconsistent with the findings for the complete blink sample, but somewhat fewer bright white dwarfs are found than expected. Weak support is also found for the occurrence of shell burning in very hot white dwarfs. 37 UV excess QSO candidates from the photometry are also identified on the basis of their colors. The distribution of their integrated surface densities from B(0) = 19.5 to B(0) = 21.5 is briefly discussed.