ON THE DETERMINATION OF THE NUMBER OF O-STARS IN H-II REGIONS AND STARBURST GALAXIES

The hot star population in H II regions, H II galaxies, and starburst galaxies is often described in terms of the number of ''equivalent'' O stars of a single representative subtype and luminosity class needed to produce the ionizing luminosity deduced from the nebular recombinat ion lines in the optical spectra. In this paper we define conversion f actors eta(0), eta(1), and zeta(5000) with which the total number of O V stars and their flux contribution at 5000 Angstrom can be derived f rom the number of these ''equivalent'' stars. These quantities depend primarily on three parameters: the slope and upper mass limit of the s tellar mass function and the metallicity of the region. Using the late st stellar atmosphere and evolution models, we calculate eta(0), eta(1 ), and zeta(5000) for a large number of values of these parameters. Th e results are presented in tabular as well as graphical form. We apply our results to two H II regions for which the hot star population are known and find that the predicted numbers of O stars agree well the o bserved counts. In addition, we describe a method by which the values of eta(0) and eta(1), and the observed emission-line fluxes can be use d to place constraints on the allowed values of the slope and upper ma ss limit of the stellar mass function in a region.