Star Counts in the Globular Cluster ω Centauri. I. Bright Stellar Components

We present a photometric investigation on HB, RGB, and MSTO stars in omega Cen= NGC 5139. The center of the clusterwas coveredwith a mosaic of F435W, F625W, and F658Nband data collectedwith HSTACS. The outer reaches were coveredwith a mosaic of U-, B-, V-, and I- band data collected with the 2.2mESO/ MPI telescope. The final catalog includes similar to 1.7million stars. We identifiedmore than 3200 likelyHBstars, 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 similar to 30% to similar to 21% when moving from the center toward the outer reaches of the cluster, while the fraction of less hot HB stars increases from similar to 62% to similar 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 similar to 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 ( Y= 0.33, 0.42) stars. We found that the observed RG/ MSTO ratio agreeswith 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 similar to 24% ( Y= 0. 42) and 30% ( Y =0.33) larger than predicted lifetime ratios.