THE STELLAR POPULATIONS OF M31

Abstract

ABSTRACT M 31 = NGC 224 are reviewed. A distance modulus (m - M)0-24.3±0.1 (estimated error), corresponding to a distance of 725 + 35 kpc, and a foreground reddening EB-V_= 0.08 will be adopted throughout this paper. Population I and Population II distance indicators give distances to M 31 that agree to better than 5%. The high rotational velocity and large velocity dispersion observed in the semistellar nucleus of the Andromeda nebula demonstrate that it contains a compact mass of ~107Msun_. Spectroscopic observation of line and molecular features now rules out the possibility that the high mass-to-light ratio observed in the nucleus of M 31 is due to a dwarf- enriched mass spectrum of star formation. Such observations also militate against the hypothesis that the dominant stellar population of the nucleus consists of stars that once belonged to globular clusters that were captured by tidal friction. The semistellar nucleus of M 31 and its dynamical center appear to be separated by ~0.5''(1.8 pc). The halo of M 31 is found to be rich in RR Lyrae stars that belong to Oosterhoff Type I. The mean metallicity of stars in the halo remains controversial. The color-magnitude diagram of the halo of M 31 appears to show that the blue horizontal-branch population is weak.