Star Counts in Omega Cen, Part I

Omega Centauri (NGC 5139) :: New Preprint

Star Counts in the Globular Cluster Omega Centauri.

I. Bright Stellar Components

Authors: V. Castellani, A.Calamida, G. Bono, P.B. Stetson, L.M. Freyhammer, S. Degl’Innocenti,
P. Prada Moroni, M. Monelli, C.E. Corsi, M. Nonino, R. Buonanno, F. Caputo, M. Castellani,
M. Dall’Ora, M. Del Principe, I. Ferraro, G. Iannicola, A.M. Piersimoni, L.Pulone, C. Vuerli

Comments: 54 pages, 17 figures,to be published in ApJ, see link at this http URL



We present an extensive photometric investigation on Horizontal Branch (HB), Red Giant Branch (RGB), and Main-Sequence Turn-Off (MSTO) stars in the Galactic globular cluster Omega Centauri = NGC 5139. The central regions of the cluster were covered with a mosaic of F435W, F625W, and F658N-band data collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. The outer reaches were covered with a large set of U,B,V,I-band data collected with the mosaic CCD camera available at the 2.2m ESO/MPI telescope. The final catalogue includes ~ 1.7 million stars. We identified more than 3,200 likely HB stars, the largest sample ever collected in a globular cluster, and more than 12,500 stars brighter than the subgiant branch and fainter than the RGB bumps (15 < B < 18). 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%. Current findings seem to support the evidence brought forward by Castellani et al. (2006a) that the Blue Tails, if affected by cluster dynamics, should be considered more a transient phenomenon rather than an intrinsic feature of GCs (…)

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