A COLOR-MAGNITUDE STUDY OF THE GLOBULAR-CLUSTER M15

B and V CCD images of the metal-poor Galactic globular cluster M 1 5 have been obtained from the Canada-France-Hawaii Telescope and the Kitt Peak National Observatory 0.9 m telescope. A complete color-magnitude diagram from the upper red-giant branch to the lower main sequence (V(lim) approximately 24) has been constructed with homogeneous calibration using the faint M15 standards from Stetson and Harris [AJ, 96, 909 (1988)]. Fitting of the CMD fiducial sequence to the Bergbusch and VandenBerg oxygen-enhanced isochrones with an objective statistical method yields an absolute age of 15 +/- 3 Gyr. This age is reduced by up to 3 Gyr if isochrones employing helium, diffusion (Proffitt and VandenBerg) are used. A differential age determination shows that M15 is (4 +/- 4) % older than M92, for a reddening of E(B-V) (M15)=0.10 +/- 0.01. A combination of techniques (including theoretical models and the local subdwarf sequence) have been employed to determine a heliocentric distance modulus (m-M)V = 15.40 +/- 0.15 (d = 10.4 +/- 0.8 kpc). A luminosity function to M(V) approximately 8.6 for the cluster main sequence has been derived; the corresponding mass function has a slope x of 1.0 +/- 0.4, which is consistent with that found in other clusters of similar metallicity and states of dynamical evolution. The derived mass function slope is also consistent with values from recent post-core-collapse models of M15.