The Upper Initial Mass Function from Ultraviolet Spectral Lines
  The space-ultraviolet wavelength region contains strong spectral lines from
massive, hot stars. These features form in winds and are sensitive to
luminosity and mass, and ultimately provide constraints on the initial mass
function. New radiation-hydrodynamical models of stellar winds are used to
construct a theoretical spectral library of massive stars for inclusion in
population synthesis. The models are compared to observations of nearby star
clusters, of starburst regions in local galaxies, and of distant star-forming
galaxies. The data are consistent with a near-universal Salpeter-type initial
mass function. We find no evidence of environmental effects on the initial mass
function. Some model deficiencies are identified: stellar rotation and binary
evolution are not accounted for and may become increasingly important in
metal-poor systems.
