UIT - NEW ULTRAVIOLET STELLAR PHOTOMETRY AND SURFACE BRIGHTNESS PROFILES OF THE GLOBULAR-CLUSTER-M79 (NGC-1904)

Ultraviolet photometry is presented for the globular cluster M79 (NGC 1904) according to the final calibration of the images obtained on the Astro-1 Spacelab mission by the Ultraviolet Imaging Telescope. These results include both point-source and surface photometry at 152 nm and 249 nm. An ultraviolet color-magnitude (C-M) diagram in these bandpas ses is presented. The detected stars are also cross-matched with the g round-based photometry of Ferraro et al. to produce C-M and two-color diagrams covering a 4000 Angstrom span in wavelength. The observed hor izontal branch (HE) is compared with stellar interior models of the ze ro-age HE (ZAHB). Many stars are found in post-HE evolutionary stages more luminous than the ZAHB. The distribution of stars along the ZAHB in m(152)-m(249) color is investigated, and an indication of possible bimodality is found. The relative population of the blue HE tail may v ary with radius for radii greater than 1'. Ultraviolet surface brightn ess and color-index profiles are presented. The previously reported ce ntral gradient in the m(152)-V color index is confirmed, and a faint d iffuse component is detected outside a radius of 40('') in the 152 nm band. Two possible causes of the central color gradient are discussed: mass segregation, and the destruction of red giant envelopes by enhan ced mass loss in the cluster core. (C) 1996 American Astronomical Soci ety.