On the Anomalous Red Giant Branch of the Globular Cluster ω Centauri
Sterken, C.; Prada Moroni, P. G.; Degl'Innocenti, S.; Pulone, L.; Stetson, P. B.; Bono, G.; Monelli, M.; Buonanno, R.; Corsi, C. E.; Ferraro, I.; Iannicola, G.; Dall'Ora, M.; Calamida, A.; Storm, J.; Freyhammer, L. M.; Petersen, J. O.; Andersen, M. I.; Cunti, P.; Del Principe, M.; Piersimoni, A.
Belgium, Spain, Italy, Canada, Germany, Denmark
Abstract
We present three different optical and near-infrared (NIR) data sets for evolved stars in the Galactic globular cluster ω Centauri. The comparison between observations and homogeneous sets of stellar isochrones and zero-age horizontal branches provides two reasonable fits. Both of them suggest that the so-called anomalous branch has a metal-intermediate chemical composition (-1.1<=[Fe/H]<=-0.8) and is located ~500 pc beyond the bulk of ω Cen stars. These findings are mainly supported by the shape of the subgiant branch in four different color-magnitude diagrams (CMDs). The most plausible fit requires a higher reddening, E(B-V)=0.155 versus 0.12, and suggests that the anomalous branch is coeval with the bulk of ω Cen stellar populations to within empirical and theoretical uncertainties. This result is supported by the identification of a sample of faint horizontal-branch (HB) stars that might be connected with the anomalous branch. Circumstantial empirical evidence seems to suggest that the stars in this branch form a clump of stars located beyond the cluster.
Based on observations made with the ESO Telescopes at the La Silla and Paranal Observatories, program IDs 63.L-0686, 64.N-0038, and 68.D-0545.