The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4

The deepest optical image ever in a globular star cluster, a Hubble SpaceTelescope (HST) 123 orbit exposure in a single field of Messier 4, was obtainedin 2 filters (F606W, F814W) over a 10 week period in early 2001. A somewhatshallower image obtained in 1995 allowed us to select out cluster and fieldobjects via their proper motion displacement resulting in remarkably cleancolor-magnitude diagrams that reach to V = 30, I = 28. The cluster mainsequence luminosity function contains very few stars fainter than MV = 15.0, MI= 11.8 which, in both filters, is more than 2 magnitudes brighter than ourlimit. This is about the faintest luminosity seen among field Population IIsubdwarfs of the same metallicity. However, there remains a sprinkling ofpotential cluster stars to lower luminosity all the way down to our limitingmagnitudes. These latter objects are significantly redder than any knownmetal-poor field subdwarf. Comparison with the current generation oftheoretical stellar models implies that the masses of the lowest luminositycluster stars observed are near 0.09 Msun. We derive the mass function of thecluster in our field and find that it is very slowly rising towards the lowestmasses with no convincing evidence of a turnover even below 0.1 Msun. Theformal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter of 2.35) witha 99% confidence interval 0.55 - 1.05. A consistency check between these slopesand the number of observed cluster white dwarfs yields a range of possibleconclusions, one of which is that we have indeed seen the termination of thewhite dwarf cooling sequence in M4.Comment: 12 pages, 3 diagrams. To be published in the Astrophysical Journal Letters August 2002. Several minor edits and a different figure 2 from original submissio