A COMPARISON OF DEEP HST LUMINOSITY FUNCTIONS OF 4-GLOBULAR-CLUSTERS

From deep color-magnitude arrays made from V and I images taken with H ubble Space Telescope's WFPC2 camera we have determined luminosity fun ctions (LFs) down to a level that corresponds to similar to 0.13 M., f or the low-metal-abundance globular clusters M15, M30, M92, and NGC 63 97. Because of the similarity of the metallicities of these clusters, differences in their luminosity functions directly trace differences i n their mass functions. The LFs of M15, M30, and M92 agree closely ove r the entire observed range, whereas that of NCC 6397 drops away sharp ly at the faintest magnitudes. We suggest that the deficiency of low-m ass stars in NGC 6397 is due to tidal shocks, to ejection through inte rnal relaxation, or to a combination of the two. With the presently av ailable mass-luminosity relations, we find that even in M15, M30, and M92 the mass functions probably do not rise so fast as to make the low -mass end dominant. (C) 1997 American Astronomical Society.