Star counts in the globular cluster omega Centauri. I. Bright stellar components

We present a photometric investigation on HB, RGB, and MSTO stars in ω Cen=NGC 5139. The center of the cluster was covered with a mosaic of F435W, F625 W, and F658N band data collected with HST ACS. The outer reaches were covered with a mosaic of U-, B-, V-, and I-band data collected with the 2.2 m ESO/MPI telescope. The final catalog includes ∼1.7 million stars. We identified more than 3200 likely HB stars, the largest sample ever collected in a globular cluster. We found that the HB morphology changes with the radial distance from the cluster center. The relative number of extreme HB stars decreases from ∼30% to ∼21% when moving from the center toward the outer reaches of the cluster, while the fraction of less hot HB stars increases from ∼62% to ∼72%. The comparison between theory and observations indicates that the empirical star counts of HB stars are on average larger (30%-40%) than predicted by canonical evolutionary models. Moreover, the rate of HB stars is ∼43% larger than the MSTO rate. We also compared theory and observations by assuming a mix of stellar populations made with 70% of canonical He (Y = 0.23) stars and 30% of He-enhanced (7 = 0.33, 0.42) stars. We found that the observed RG/MSTO ratio agrees with the predicted lifetimes of He-mixed stellar populations. The discrepancy between theory and observations decreases by a factor of 2 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 ratios between HB and MSTO star counts are ∼24% (Y = 0.42) and 30% (Y = 0.33) larger than predicted lifetime ratios. © 2007. The American Astronomical Society. All rights reserved