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H.B. Richer, J. Brewer (UBC), G.G. Fahlman (CFHT), B. Gibson (Swinburne U.), B. Hansen (Princeton), R. Ibata (Strasbourg), M. Limongi (Rome Obs.), M.R. Rich (UCLA), P.B. Stetson (HIA), M. Shara (AMNH)
A 123 orbit exposure with HST in 2 colors (GO-8679) in a single field of M4 has yielded photometry to \rm V ~ 30, \rm I ~28.5. In the proper motion selected cluster color-magnitude diagram constructed from these and earlier epoch data, there is an abrupt termination of the cluster main sequence at about \rm V = 28 (\rm MV = 15.5), fully 2 magnitudes brighter than our limiting magnitude. This is similar to what is seen among field Population II subdwarfs of the same metallicity (\rm [m/H] = -1.3) and suggests that the lowest mass stars capable of burning hydrogen in their cores at this metallicity have been detected. Comparison with the current generation of models suggests that the masses here are near \rm 0.09 M\odot. Of particular interest are 6 objects which have proper motions appropriate for cluster membership, are moderately bright, but are undetected in V. The brightest of these will have \rm (V - I) > 4, redder than any known metal poor subdwarf. The nature of these objects is currently unknown but one possibility is that they are recycled brown dwarfs.