Hubble Space Telescope / Faint Object Camera Observations of the Inner Regions of the Galactic Globular Cluster M15

Abstract

A set of high resolution images of the core of the Galactic Globular Cluster M15 (NGC 7078) were obtained for the first time with the Faint Object Camera on the HST through the f/48-F140W, f/48-F220W and f/96-F342W ultraviolet filters. Two UV-Color-Magnitude Diagrams are presented for the approximately 300 objects measured in the 22 arcsec x 22 arcsec field centered on the core covered by our observations. Out of these, 19 are variables discovered from the rms frame-to-frame scatter of the F220W images, 15 of which are probably RR Lyrae. The existence of the gap on the Horizontal Branch (HB) separating the bulk of the HB stars from the Blue Tail already known for the outer parts of the cluster, is confirmed for the core regions. The temperature and the mass distribution of the HB stars is derived from the ultraviolet color-color diagram. We also detect a well developed Blue Straggler sequence. At least two of the brighter blue stragglers reside within the sharp core.