THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 99:173-187, 1995 July

      ATLAS OF SYNTHETIC ULTRAVIOLET SPECTRA OF MASSIVE STAR POPULATIONS

                      CLAUS LEITHERER AND CARMELLE ROBERT
           Space Telescope Science Institute, 3700 San Martin Drive,
          Baltimore, MD 21218; leitherer@stsci.edu, crobert@stsci.edu

                                      AND

                              TIMOTHY M. HECKMAN
          Physics and Astronomy Department, Johns Hopkins University,
           Homewood Campus, Baltimore, MD 21218; heckman@pha.jhu.edu


                                   ABSTRACT

        An atlas of synthetic ultraviolet spectra of a population of
     massive stars is presented.  The spectra are based on a stellar
     library of IUE high-dispersion spectra of O and Wolf-Rayet stars,
     coupled to an evolutionary synthesis code.  Later spectral types are
     included via low-dispersion spectra.  Line profiles of N V lambda
     1240, Si IV lambda 1400, C IV lambda 1550, He II lambda 1640, and N IV
     lambda 1720 have been computed for star-formation histories and
     initial mass functions typically found in starburst regions.
        It is found that the lines are sensitive indicators for the
     presence or absence of massive stars.  C IV lambda 1550 is the
     strongest stellar line in the ultraviolet spectrum of a typical
     starburst.  If O stars with zero-age main-sequence masses above 50
     Msun are present, C IV always shows a P Cygni profile.  In the absence
     of such stars, only a blue-shifted absorption is present.  During
     later epochs of the starburst, when late-O/early-B stars dominate, an
     unshifted photospheric absorption appears.
        Si IV lambda 1400 shows a conspicuous wind profile when luminous O
     supergiants are present.  A strong P Cygni profile is found only for
     an instantaneous burst observed at 3 to 5 Myr, which has a top-heavy
     IMF.  The velocity of the blueshifted absorption is strongly
     correlated with the age and the upper cutoff mass (or slope) of the
     IMF.
        N V lambda 1240 traces the most massive stars and behaves rather
     similar to Si IV lambda 1400.  Its usefulness as an indicator of very
     massive stars is limited due to the strong blending effect of the
     nearby Ly-alpha line.  Nevertheless, strong N V lambda 1240 emission
     in a starburst suggests the presence of stars with masses in excess of
     60 Msun.
        He II lambda 1640 and N IV lambda 1720 are produced by very hot and
     luminous O and Wolf-Rayet stars.  Both lines can have weak absorption
     or emission in a typical starburst but are predicted to be observable
     only under rare circumstances, such as in an instantaneous burst at
     t approx. 3 Myr.
        The profiles presented in the atlas can be compared to high-quality
     ultraviolet observations of galaxies with active star formation in
     order to constrain the massive star population.  The atlas is
     published in its entirety in computer-readable form in the AAS CD-ROM
     series, Vol. 5.

     Subject headings: atlases -- stars: early-type -- stars: mass-loss --
                       stars: Wolf-Rayet -- ultraviolet: stars