The mass function of M 4 from near IR and optical HST observations
Abstract
Deep images of the galactic globular cluster M 4 taken at various locations with the NIC 3 and the WFPC 2 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 powerlaw dN ~ m(x) dlog m, with single exponent x = 0.8 for the inner regions and a twosegment powerlaw which rises with x = 1 down to M_{814} =~ 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 multimass KingMichie 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 in [1.4, 1.9] and r_c in [50{('') } , 90{('') } ]. The best compromise, in this case, consists in a model with a twosegment powerlaw mass function with exponents x = 0.21.0 in the mass interval 0.8 < M_{sun} < 0.25, x = 0.4 for m <= 0.25 M_sun 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 ~ 0.25 M_sun and then flattening out and possibly dropping to the Hburning limit. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS526555
 Publication:

Astronomy and Astrophysics
 Pub Date:
 February 1999
 DOI:
 10.48550/arXiv.astroph/9811307
 arXiv:
 arXiv:astroph/9811307
 Bibcode:
 1999A&A...342..440P
 Keywords:

 STARS: HERTZSPRUNGRUSSEL (HR) AND CM DIAGRAMS;
 STARS: LUMINOSITY FUNCTION;
 MASS FUNCTION;
 GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL: M 4;
 Astrophysics
 EPrint:
 14 pages, 17 figures, to appear in Astronomy and Astrophysics. corrected some typos on Dec 17, 1998