A photometric study of the globular cluster M55

We present new V, I photometry of a field 2.5 core radii from the center of the metal-poor Galactic globular cluster M55. Our data reaches M(V) approximate to 11 and M(I) approximate to 9.3, where the photometry is complete at the 50% level. We drive an apparent distance modulus of (m-M)(V)=13.90+/-0.09 by fitting the fiducial main sequence of M55 to an expanded and improved sample of nearby subdwarfs with well-determined parallaxes. The location and the shape of the main-sequence turnoff and the subgiant branch are matched superbly by a 16-Gyr, alpha-enriched isochrone with [Fe/H]=-1.84, Y=0.236 and [alpha/Fe]=0.3. We present also V-band and I-band luminosity functions and a mass function which extends to similar to 0.2H.. The luminosity function levels off at M(V) approximate to 9.8 (M(I) not similar or equal to 8.3) and remains flat to the faint limit of the photometry. We find that the mass function (MF) of M55 in this field rises steeply for masses below similar to 0.35M. and that it cannot be described by a single power law of the form N(M) alpha M(-(1+x)): at the high-mass end (0.5 less than or similar to M/M.less than or similar to 0.8) the MF is rather shallow and is approximated will by a power law with x=0.5+/-0.2, whereas at the low-mass end (M/M.<0.4) it has a slope x=1.6+/-0.1. (C) 1996 American Astronomical Society.