THE ASTRONOMICAL JOURNAL    VOLUME 112, NUMBER 2, PAGE 601          AUGUST 1996

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

         ROBERT S. HILL,[1] K.-P. CHENG,[2,3,4] ERIC P. SMITH,[3,4,5]
   PAUL M. N. HINTZEN,[3,4,5,6] RALPH C. BOHLIN,[7] ROBERT W. O'CONNELL,[8]
     MORTON S. ROBERTS,[9] ANDREW M. SMITH,[5] AND THEODORE P. STECHER[10]


                                   ABSTRACT

    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
    bandpasses is presented.  The detected stars are also cross-matched
    with the ground-based photometry of Ferraro et al. to produce C-M and
    two-color diagrams covering a 4000 A span in wavelength.  The observed
    horizontal branch (HB) is compared with stellar interior models of the
    zero-age HB (ZAHB).  Many stars are found in post-HB evolutionary
    stages more luminous than the ZAHB.  The distribution of stars along
    the ZAHB in m152-m249 color is investigated, and an indication of
    possible bimodality is found.  The relative population of the blue HB
    tail may vary with radius for radii greater than 1'.  Ultraviolet
    surface brightness and color-index profiles are presented.  The
    previously reported central gradient in the m152-V color index is
    confirmed, and a faint diffuse 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 enhanced mass loss in the cluster core.

[1] Hughes STX Corp., Code 681, NASA/Goddard Space Flight Center,
    Greenbelt, MD 20071.
[2] Department of Physics, California State University, Fullerton, CA
    92634.
[3] Visiting Observer at the Kitt Peak National Observatory of the National
    Optical Astronomy Observatories operated by Associated Universities for
    Research in Astronomy, Inc. under cooperative agreement with the
    National Science Foundation.
[4] Visiting Observer at the Cerro Tololo Inter-American Observatory of the
    National Optical Astronomy Observatories operated by Associated
    Universities for Research in Astronomy, Inc. under cooperative
    agreement with the National Science Foundation.
[5] Laboratory for Astronomy and Solar Physics, Code 681, NASA/GSFC,
    Greenbelt, MD 20771.
[6] Department of Physics and Astronomy, University of Nevada, Las Vegas,
    NV 19154.
[7] Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD
    21218.
[8] University of Virginia, P.O. Box 3818, Charlottesville, VA 22903.
[9] National Radio Astronomy Observatory, Edgemont Rd., Charlottesville, VA
    22903.
[10] Laboratory for Astronomy and Solar Physics, Code 680, NASA/GSFC,
    Greenbelt, MD 20771.