The initial mass function for massive stars.

A machine readable catalog of over 750 galactic O stars with published photometry, spectral types, and luminosity classes has been compiled. The catalog is probably complete to a distance of about 2.5 kpc. From this volume-limited data, the initial mass function (IMF) for stars more massive than 20 solar masses has been derived. This IMF differs from that of Miller and Scalo (1979) and of Lequeux (1979), in having proportionately more O type stars and not as steep a fall-off in number of stars with increasing mass. Dividing the sample into stars inside and outside the solar circle, a substantial difference in the IMF of the most massive stars is found. There are proportionally more toward the galactic center. This gradient in the IMF may be related to the observed space density of Wolf-Rayet stars, which are descendants of O type stars.