The lower main sequence of the globular cluster M3 with the Hubble Space Telescope: luminosity and mass functions

Abstract

We present HST observations of the lower main sequence of the Galactic globular cluster M3. Our data, obtained with WFPC2 in the F814W and F555W filters, sample a region centred about 4 arcmin away from the cluster centre (at ~4.2 core radii). The photometry of the WF2, WF3 and WF4 fields extends down to V~26.5 (I~25), much deeper than any previous ground-based study on this cluster. The resulting luminosity function (LF) for the main sequence covers the interval M_I=3-10.5, and reaches a maximum between M_I=8.5 and 9, sharing in this feature the same behaviour as other clusters studied with HST so far. Making use of four different mass-luminosity (M-L) relations (theoretical and semi-empirical) a present-day mass function (MF) has also been obtained, which spans the mass interval ~=0.8-0.18 Msolar. Independent of the adopted M-L relation, the MF clearly flattens out at the low-mass limits. However, no attempt has been made to take into account possible effects due, for instance, to mass segregation and evaporation.