Astrophys. J., 663, 1021-1039 (2007/July-2)
Star counts in the globular cluster ω Centauri. I. Bright stellar components.
CASTELLANI V., CALAMIDA A., BONO G., STETSON P.B., FREYHAMMER L.M., DEGL'INNOCENTI S., PRADA MORONI P., MONELLI M., CORSI C.E., NONINO M., BUONANNO R., CAPUTO F., CASTELLANI M., DALL'ORA M., DEL PRINCIPE M., FERRARO I., IANNICOLA G., PIERSIMONI A.M., PULONE L. and VUERLI C.
Abstract (from CDS):
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, F625W, 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 (Y=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.
Galaxy: Globular Clusters: Individual: Name: ω Centauri - Stars: Evolution - Stars: Horizontal-Branch
VizieR on-line data:
<Available at CDS (J/ApJ/663/1021): table1.dat>
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