A search for UV excess objects in the direction of the globular cluster NGC 6121 (M4)

A blink survey was performed on a 50' diameter field centered about the globular cluster M4. The purpose was to select out possible cluster white dwarfs and UV excess QSOs in the field. A total of 1096 objects brighter than U = 22.3 were found in the region between 3.9' and 23.7' from the cluster center. Subtraction of the expected background revealed an obvious concentration of the objects towards the center of the cluster. The radial gradient matches well with that obtained by King et al. (1968) for their star counts in M4. This gives a lower limit of ≈ 240 white dwarfs in M4 brighter than M[sub v] ≈ 11. Photometry of 74 of the objects which were visible on both the blinked U and B plates yielded 9 objects with colours and photometric parallaxes expected for cluster white dwarfs. Five of these have M[sub v]< 8.5, assuming that they are indeed in the cluster. Comparisons of these numbers have been made with theoretical estimates from Fusi-Pecci and Renzini (1979) and from Hills (1974) and were not found to be inconsistent with the predictions. A total of 37 UV excess QSOs were also identified on the basis of their colours. Their integrated surface densities from B₀ = 19.5 to B₀ = 21.5 correspond well with the pure luminosity evolution model of Bracessi et al. (1980).Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat