THE INTERMEDIATE-MASS STELLAR POPULATION OF THE LARGE MAGELLANIC CLOUD CLUSTER NGC-1818 AND THE UNIVERSALITY OF THE STELLAR INITIAL MASS FUNCTION

We present stellar photometry from Hubble Space Telescope images of NG C 1818, a young populous star cluster in the Large Magellanic Cloud. T he cluster stars in both the core and the outlying regions are well re solved, and the photometry extends to a V magnitude of 26, correspondi ng approximately to a K4 V star. With the use of isochrones, we comput e a stellar initial mass function (IMF) for stars from 0.85 to 9 M.. T he slope of the mass function is -1.23 +/- 0.08, which is close to the Salpeter (1955) slope of -1.35. The NGC 1818 star cluster represents a star forming event intermediate between that of open clusters and of globular clusters in terms of the mass of stars formed and their spat ial concentration. The products of the star forming event itself, as a diagnostic of the physical processes, indicate that star formation in NGC 1818 proceeded in a manner similar to that in events that are bot h less and more concentrated or rich in stars. We compare IMF slopes t hat have been measured from star counts in clusters and associations i n Local Group galaxies, and we conclude that for young stellar cluster s and associations the IMF is independent of the spatial concentration of the stars formed, the richness of stars formed, galactic character istics including metallicity, and, at least down to 0.85 M., the stell ar mass range.