THE MASSIVE STAR CONTENT, REDDENING, AND DISTANCE OF THE NEARBY IRREGULAR GALAXY IC-10

IC 10 is a nearby dwarf irregular galaxy that contains a large number of candidate Wolf-Rayet stars and has an uncertain distance modulus and an uncertain, but large, reddening. In this paper, we present spectroscopic observations of the IC 10 Wolf-Rayet candidates. In addition, CCD BV photometry of stars in IC 10 is used to construct a color-magnitude diagram in order to constrain the reddening and distance of the galaxy. Spectroscopy of the Wolf-Rayet candidates confirms 15, giving IC 10 a galaxy-wide surface density of massive stars that is higher by a factor of 2 than any other Local Group galaxy, and is comparable to that observed in isolated regions of recent star formation. This is in accord with suggestions that IC 10 is undergoing a starburst. Most of the Wolf-Rayet stars are of the WC type, giving IC 10 a similarly anomalous ratio of WC-type to WN-type stars, contrary to predictions that this quantity simply depends on metallicity. Photometry of the Wolf-Rayet stars and the bright, blue stars lead to similar estimates of the reddening [E(B-V)=0.75-0.80] and distance [(m-M)0=24.9]. Comparison with recent literature values suggests that a consensus is developing that IC 10's distance is ≈1 Mpc. With the additional knowledge provided by IC 10, an understanding of the variations seen in the WC/WN ratio among the galaxies of the Local Group is emerging. We argue that the initial mass function and metal abundance together determine the WC/WN ratio. We speculate that IC 10's large WC/WN ratio at a low metal abundance can be explained if its initial mass function has been skewed toward high-mass stars due to the very vigorous star formation that is occurring. © 1995 American Astronomical Society.