1999A&A...342..440P

Deep images of the galactic globular cluster M4 taken at various locations with the NIC3 and the WFPC2 cameras on HST were used to derive detailed local optical and near IR luminosity functions. White dwarfs have been detected for the first time on a color sequence at constant luminosity in the F110W band. Transformation of the observed luminosity functions into mass functions via the most up to date theoretical mass luminosity relations currently available results in best fit local mass functions, in logarithmic mass units, that consist of a power-law dN ∝m^–x dlogm, with single exponent x=-0.8 for the inner regions and a two-segment power-law which rises with x=1 down to M₈₁₄≃8.5 and then drops all the way to the detection limit with x=-0.3 for the outer regions. This behaviour cannot be reconciled with the expectations of a multi-mass King-Michie dynamical model using as input the canonical structure parameters for this cluster (core radius r_c=50" and concentration c=1.6: Harris, 1996). Thus, either the model does not accurately reflect the structure of the cluster due to some effect not properly accounted for in it or the canonical cluster structural parameters have to be significantly modified. Reasonable fits to all the present observations can be obtained with various global mass functions provided the cluster's structural parameters such as concentration c and core radius r_c, are in the range c ∈[1.4, 1.9] and r_c ∈[50", 90"]. The best compromise, in this case, consists in a model with a two-segment power-law mass function with exponents x=0.2-1.0 in the mass interval 0.8<M_☉<0.25, x=-0.4 for m≤0.25M_☉ and structural parameters that require the least modification from the currently established values. This last result differs only minimally from that obtained for other globular clusters studied so far with HST which seem to have global mass functions increasing up to a peak at ∼0.25M_☉ and then flattening out and possibly dropping to the H-burning limit.