$\omega$ Centauri

People involved at OAB: Bellazzini, Origlia, Pancino, Sollima

$\omega$ Centauri is the largest ( $M=4\times 10^6 M_{\odot}$, Pryor & Meylan 1993), brightest cluster in the Galactic Halo, and it is the only GC which shows undisputed variations in the chemical content of its stars. From this point of view, $\omega$ Cen could be considered a bridge system between genuine globulars, which are unable to retain the gas ejected by their former massive stars, and dwarf galaxies, which are the least massive self-enriching stellar systems known. We are currently carrying out a long-term project, aimed at performing a detailed photometric and spectroscopic study of the stellar population in this cluster.

The first surprising result was the discovery of a distinct, anomalous RGB (RGB-a), significantly redder than the bulk of the normal RGB stars (Pancino et al. 2000). Prompted by this result, we have then carried out a systematic spectroscopic and photometric observational campaign, using the current generation of instruments available at ESO, on board of HST and at other international telescopes. A spectroscopic screening of the multi-populations of red giants in $\omega$ Cen has been carried out by means of high-resolution echelle spectra with UVES@VLT (Pancino 2003, PhD thesis) and medium-resolution infrared spectra with SOFI@NTT of more than 40 stars in 2002 (see Origlia et al. 2003 and references therein). During 2003 we have acquired more than 700 high-resolution spectra of $\omega$ Cen giants with UVES and FLAMES, within the GTO programmes of the ITAL-FLAMES consortium. The data reduction is in progress. In the meantime the results of a detailed study of the structural properties of $\omega$ Cen as a whole and of its sub-populations have been published (Pancino et al. 2003).

Figure 1: (R625, B435-R625) CMD for more than 400,000 stars identified in the nine ACS fields. The anomalous SGB is clearly visible, along with the complex substructures of the TO-SGB region of $\omega$ Cen.

The results of new NIR and optical photometric campaigns are in press or in preparation. In this vein the most striking result comes from deep HST-ACS photometry of the central region of the cluster (Ferraro et al. 2004) that clearly evidences for the first time substructures in the CMD at the level of the sub-giant branch (SGB, see Fig. 1). In particular, an anomalous SGB is clearly identified which is significantly fainter than that associated with the bulk of the cluster population. At present it does not seem possible to obtain a satisfying explanation of such a feature in terms of age/metallicity/He-abundance variations. We are reducing low-resolution spectra of stars in the anomalous SGB to clarify their nature. Preliminary results indicate that such stars are indeed members of the cluster and that they are quite metal-rich ( $[Fe/H]\sim -0.6$).

The Bologna key project on $\omega$ Cen is fully active, with a wealth of data being reduced, several observational proposals already accepted, one PhD student (A. Sollima) and one post-doc (E. Pancino) working full-time on this topic. The work is carried out in collaboration with F. Ferraro (Astr. Dept., Univ. of Bologna).