We present a photometric investigation on HB, RGB, and MSTO stars in Ï‰ 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 âˆ¼1.7 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 âˆ¼30% to âˆ¼21% when moving from the center toward the outer reaches of the cluster, while the fraction of less hot HB stars increases from âˆ¼62% to âˆ¼72%. 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 âˆ¼43% 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 âˆ¼24% (Y = 0.42) and 30% (Y = 0.33) larger than predicted lifetime ratios. Â© 2007. The American Astronomical Society. All rights reserved.
Castellani V., C.A. (2007). Star counts in the globular cluster omega Centauri. I. Bright stellar components. THE ASTROPHYSICAL JOURNAL, 663(2 I), 1021-1039.
|Tipologia:||Articolo su rivista|
|Citazione:||Castellani V., C.A. (2007). Star counts in the globular cluster omega Centauri. I. Bright stellar components. THE ASTROPHYSICAL JOURNAL, 663(2 I), 1021-1039.|
|Settore Scientifico Disciplinare:||Settore FIS/05 - Astronomia e Astrofisica|
|Revisione (peer review):||Sì, ma tipo non specificato|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.1086/518209|
|Stato di pubblicazione:||Pubblicato|
|Data di pubblicazione:||2007|
|Titolo:||Star counts in the globular cluster omega Centauri. I. Bright stellar components|
|Autori:||Castellani V., Calamida A., Bono G., Stetson P.B., Freyhammer L.M., Degl'Innocenti S., Moroni P.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., Vuerli C.|
|Appare nelle tipologie:||01 - Articolo su rivista|