Abstract details

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).