  We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen.
The central regions of the cluster were covered with a mosaic of F435W, F625W,
and F658N-band data collected with ACS/HST. The outer reaches were covered with
a large set of U,B,V,I-band data collected with the<EMAIL_ADDRESS>ESO/MPI telescope.
The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB
stars and ~12,500 stars brighter than the subgiant branch and fainter than the
RGB bumps. The HB morphology changes with the radial distance. The relative
number of extreme HB stars decreases from ~30% to ~21% when moving from the
center toward the outer regions of the cluster, while the fraction of less hot
HB stars increases from ~62% to ~72%. We performed a detailed comparison
between observed ratios of different stellar tracers and predictions based on
canonical evolutionary models with a primordial helium (Y=0.23) content and
metal abundances (Z=0.0002,0.001) that bracket the observed spread in
metallicity of Omega Cen stars. We found that the empirical star counts of HB
stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB
stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB
compared with the rate of RG and MSTO stars supports the evidence that we are
facing a true excess of HB stars. The same comparison was performed by assuming
a mix of stellar populations made with 70% of canonical stars and 30% of
He-enhanced stars. The discrepancy between theory and observations decreases by
a factor of two when compared with rates predicted by canonical He content
models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed.
Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and
30% (Y=0.33) larger than predicted lifetime ratios.
