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

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Abstract
The deepest optical image ever in a globular star cluster, a Hubble Space Telescope (HST) 123 orbit exposure (GO 8679, cycle 9) in a single field of Messier 4, was obtained in 2 filters (F606W - V, F814W - I) over a 10 week period in early 2001. A somewhat shallower image obtained in 1995 (GO 5461, cycle 4) allowed us to select out cluster and field objects via their proper motion displacement (PMD) resulting in remarkably clean color-magnitude diagrams (CMDs) that reach to V = 30, I = 28. The cluster main sequence (MS) luminosity function (LF) contains very few stars fainter than MV = 15.0, MI = 11.8 which, in both filters, is more than 2 magnitudes brighter than our limit. This is about the faintest luminosity seen among field Population II subdwarfs of the same metallicity. However, there remains a sprinkling of potential cluster stars to lower luminosity all the way down to our limiting magnitudes. These latter objects are significantly redder than any known metal-poor field subdwarf. Comparison with the current generation of theoretical stellar models implies that the masses of the lowest luminosity cluster stars observed are near 0.09 Msun. We derive the mass function (MF) of the cluster in our field and find that it is very slowly rising towards the lowest masses with no convincing evidence of a turnover even below 0.1 Msun. The formal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter = 2.35) with a 99% confidence interval 0.55 - 1.05. A consistency check between these slopes and the number of observed cluster white dwarfs (WDs) yields a range of possible conclusions, one of which is that we have indeed seen the termination of the WD cooling sequence in M4.

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