CCD photometry in the globular cluster M4

A V, B-V composite colour-magnitude diagram, based upon CCD photometry from V∼12 on the red giant branch to V∼25 on the main sequence is presented for the globular cluster M4. The distance to M4, from fitting local subdwarfs to the lower main sequence, is $$(m-M)_V=12.84\pm 0.19$$. The luminosity function of the cluster reaches to $$M_V\gt 11$$. At $$M_V \gt 10$$ the luminosity function shows a turnover. The mass function extends down to M∼0.19 M_⊙ and is quite flat, having a power-law index of $$x=-0.15\pm 0.04$$ over the range studied. It is suggested that the most likely explanation for the observed flatness in the mass function is that the cluster has undergone extensive tidal stripping. The best match to theoretical isochrones is for the case [Fe/H]=-1.27 and an age of 16±1 Gyr. A differential age comparison with NGC 362 shows that M4 is approximately 1.7 Gyr older. Finally, measurement of the small intrinsic colour spread on the subgiant branches indicates that there is no strong evidence for any real spread in metallicity among cluster stars.