SYNTHETIC UV LINES OF SI-IV, C-IV, AND HE-II FROM A POPULATION OF MASSIVE STARS IN STARBURST GALAXIES

We present the first results of our study of the massive star population in starburst galaxies based on UV data. We have synthesized the Si IV lambda1400, C IV lambda1550 and He II lambda1640 lines for both a continuous and an instantaneous burst of star formation with approximately solar chemical composition. Our code uses the latest generation of stellar evolutionary models, stellar atmosphere codes, and a library of high-dispersion IUE spectra of hot stars. Models were computed for various values of the IMF parameters. Si IV lambda1400 and C IV lambda1550 develop P Cygni profiles when formed in strong stellar winds from the most massive stars. The velocity shifts predicted for these lines give a tight constraint on the value of the IMF upper mass cutoff: strong blueshifts in both lines are produced if stars with an initial mass larger than 30-60 M. are included in the models. Based on the line velocity shifts, it also seems possible to put limits on the burst age. The models show only a small dependence of the line velocity shifts on the IMF slope. We also find a significant dependence of the equivalent widths of Si IV lambda1400 and C IV lambda1550 on the burst age, the IMF upper cutoff mass, and the IMF slope. The He II lambda1640 line shows a strong broad emission profile when formed in winds from evolved massive stars. If offers additional important clues to the burst age and the IMF upper cutoff mass. We have compared the model parameters with data obtained for an average galaxy spectrum formed by combining low-dispersion IUE spectra of 13 starburst galaxies with nearly solar chemical composition. The most interesting result, based on the Si IV lambda1400 and C IV lambda1550 line velocity shifts and the strength of the broad He II lambda1640 emission line, is that evolved massive stars with an initial mass larger than 30 M. must be present in most of these galaxies. We find a good fit to the data for a model of an instantaneous burst of age congruent-to 5 x 10(6) yr or a model for which star formation is proceeding at a constant rate for congruent-to 10(7) yr. Hubble Space Telescope data with higher spectral resolution will be required to test these ideas and to allow us to fully exploit our method.