The center of the core-cusp globular cluster M15: CFHT and HST Observations, ALLFRAME reductions

The central brightness cusps seen in some globular clusters are thought to be the relics of a gravothermal core collapse that occurred in the clusters' past. Recent observations show that the centers of such clusters are bluer than their outskirts, indicating that the stellar populations there are somehow different than those farther out, presumably as a result of unusual physical processes that took place in these incredibly dense regions. Here I analyze a large body of digital imagery from the Canada-France-Hawaii Telescope and the Hubble Space Telescope to obtain color-magnitude and color-color diagrams for stars in the central two arcminutes of the prototypical core-cusp globular cluster M15 = NGC 7078 = C2127+119. These data were reduced with a new computer program, named ALLFRAME, that is described in detail here for the first time. ALLFRAME makes simultaneous use of the geometric and photometric information from all images of a given field to derive a self-consistent set of positions and magnitudes for all detected star-like objects in that area of sky, thereby extending the range of magnitude and crowding conditions for which useful photometry is obtainable. I tentatively conclude that teh color gradient in M15 is due to three distinct effects: (1) there is a deficiency of the brightest red giants in the cetnral regions of the cluster; (2) the giant branch of the dominant cluster population shifts systematically toward the blue as the center of the cluster is approached; and (3) the very center of the cluster (radius </= 12") contains a large population of blue and yellow stragglers that occupy the area of the color magnitude diagram between the main-sequence turnoff and the horizontal branch and between the extended blue horizontal branch and the subgiant branch; many of these appear to have a significant ultraviolet excess.