SI-IV AND C-IV RESONANCE LINES AS INDICATORS OF MASSIVE STARS IN STARBURST GALAXIES

We investigate the use of the ultraviolet lines of Si IV and C IV to study the initial mass function of massive stars in starburst galaxies. Radiatively driven wind models of stars with different metal content are computed. The resulting wind parameters are used as an input for line-profile calculations of Si IV and C IV. We describe the metallicity dependence of the equivalent widths of these lines for representative hot star models. We show that the ratio of the Si IV and C IV line strengths decreases with decreasing metallicity under the conditions typically prevailing in stellar winds. We also show that in a combined spectrum of a galaxy, in which main-sequence stars dominate the UV spectrum, this ratio is expected to increase with decreasing metallicity for a constant initial mass function. The possible contribution of interstellar lines to the integrated spectrum is also discussed. We find that spectral synthesis models do not allow one to derive parameters of the initial mass function as has been done previously. The mass-loss characteristics of massive stars directly affect the predicted line strengths, and they enter in an indirect way by modifying the evolutionary history of the stars. Both effects are not accounted for in synthesis models presently in existence.