  Deep HST-WFPC2 observations of the stellar population in the globular cluster
Omega Cen (NGC 5139) have been used to measure the luminosity function of main
sequence stars at the low-mass end. Two fields have been investigated, located
respectively ~4.6' and ~7' away from the cluster center, or near the half-light
radius of this cluster (~4.8'). The color-magnitude diagrams derived from these
data show the cluster main sequence extending to the detection limit at I ~ 24.
Information on both color and magnitude is used to build the luminosity
functions of main sequence stars in these fields and the two independent
determinations are found to agree very well with each other within the
observational uncertainty. Both functions show a peak in the stellar
distribution around M_I ~ 9 followed by a drop at fainter magnitudes well
before photometric incompleteness becomes significant, as is typical of other
globular clusters observed with the HST. This result is at variance with
previous claims that the luminosity function of Omega Cen stays flat at low
masses, but is in excellent agreement with recent near-IR NICMOS observations
of the same cluster.
