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Symposium 241

Abstract details

Omega Centauri: a possible link between globular clusters and dwarf spheroidal galaxies
A. Calamida, G. Bono, R. Buonanno, C.E. Corsi, L.M.Freyhammer, F. Grundahl, M. Monelli, A. Piersimoni, P.B. Stetson

Abstract
This Ph.D. project is focused on $\omega$ Cen. This stellar system might be the link between dwarf galaxies and globular clusters, and is the only one that shows a well-defined spread in metallicity, helium abundance, age and probably differential reddening (Piotto et al. 2005). Moreover, it has been suggested the existence of tidal tails along the $\omega$ Cen orbit, but this result is still controversial. For all these peculiar features, $\omega$ Cen is supposed to be the core of a dwarf galaxy, that lost most of its stellar content. In order to understand the origin of this stellar system, we undertook a multiwavelength photometric study based on both groud-based and HST data. The accurate ACS {\it BRH$_a$\/} photometry of the cluster center, will allow us to disentangle the different stellar populations and to provide robust estimates of star counts, while the broad-band WFI {\it UBVI\/} photometry to study their radial distributions across the cluster field. By using medium-band {\it uvby\/} Str\"omgren~ photometry for the central regions of $\omega$ Cen, and a metallicity-independent, reddening-free temperature index for hot horizontal-branch (HB) stars ($[c]$), we found clumpy extinction variations of almost a factor of two within the area of the cluster core (Calamida et al. 2005). Moreover, we plan to obtain metallicity estimates for red-giant stars in $\omega$ Cen, using a new calibration of the Str\"omgren~ metallicty indicator $m_1$. We are also analysing time-series Danish {\it BVI\/} photometry ($\sim$5000 images), covering a time interval of at least 10 years, which will allow us to investigate the long-term behavior of regular variables (RRLyrae, $\delta$ Scuti, Sx Phenicis, Type II Cepheids, Semi-regulars), and to identify both short (days) and long (months) period binaries in $\omega$ Cen. The complete census of bright binaries will allow to costrain the dynamical status of the cluster and the role that binaries might play in several long-standing problems: HB morphology and second parameter, occurrence of He-core white dwarfs (Monelli et al. 2005).