Radial Density Profiles in the Centers of 47 TUC and M15 from HST Images
Abstract
Hubble Space Telescope Planetary Camera (HST/PC) images of the centers of the dense Galactic globular clusters 47 Tuc and M 15 are presented. We study the radial dependence of the surface density of post-main sequence stars and the surface brightness of the diffuse light of faint unresolved stars below the main sequence turnoff. The radial profile of 47 Tuc is fit well by a King model with core radius r_c=23'' (0.5 pc). The extremely high density of stars near the center of post-core collapse cluster M 15 and the broad wings of the point spread function (PSF) caused by spherical aberration in HST's primary mirror combine to make stellar photometry a painstaking process A technique, specially designed to work in crowded fields, has been used to derive an empirical PSF model and make a fit to the stellar images. Accurate fitting and subtraction of the light of bright red giant stars and estimating the incompleteness as a function of radius and magnitude are crucial to this study. Realistic simulated images have been constructed and analyzed for this purpose. We compare the M 15 data to simulations with a small, but finite core (r_c=2''=0.1pc) and those with mild central cusps (surface density ~ r(-alpha ) , where alpha =0.5-0.8).
- Publication:
-
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1992
- Bibcode:
- 1992AAS...18110802Y