$\omega$ Centauri

People involved at OAB: Bellazzini, Cacciari, Origlia, Pancino, Romano, Tosi.

$\omega$ Centauri is the largest ( $M=2.9\times 10^6 M_{\odot}$, Merrit et al. 1997), brightest cluster in the Galactic Halo, and surely the most peculiar one in terms of structure, kinematics and stellar contents. It is in fact 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.

Recent photometric surveys have revealed the presence of several anomalous sequences in the CMD indicating a complex star formation and chemical enrichment history for $\omega$ Centauri (Pancino et al. 2001). For instance, Bedin et al. (2004) discovered a bifurcation in the MS of the cluster. Although the location of the observed blue Main Sequence (bMS) would suggest a low metallicity, spectroscopic studies indicate that it has a high metal abundance (Piotto et al. 2005). Hence, it cannot be associated to the dominant metal poor population. At present it does not seem possible to obtain a satisfying explanation of such a feature without assuming large He-abundance variations.

Our group is carrying on a long-term multi-approach programme to investigate the nature and the evolution of this fascinating stellar system (see Ferraro et al. 2003) and has actively participated in many of the most exciting discoveries of the latest years (for example, on the anomalously red and metal-rich RBG and faint SGB), publishing a number of original results on the structural, photometric, chemical and kinematic properties of $\omega$ Cen (Pancino et al. 2000, 2002, 2003; Ferraro et al. 2002, 2004; Sollima et al. 2004, 2005a, 2005b; Origlia et al. 2003; Bellazzini et al. 2004).

In particular, during 2006:

i) Using FLAMES observations we derived updated metallicities for a sample of 74 RR Lyrae stars in this stellar system. These data, together with the photometric properties of these stars, compared with stellar evolution models have shown the presence of a metal-intermediate population which shows no evidence of a significant He overabundance (Sollima et al. 2006).

ii) We recently performed a wide field photometric survey using deep FORS1 observations in order to sample the bMS population over a large radial extent. This analysis shows that bMS stars appear to be more concentrated than the other ``normal'' cluster stars (Sollima et al. 2007). These results suggest that an inhomogeneous chemical enrichment process at the basis of the possible He enhancement occurred in some of the stellar populations of $\omega$ Cen.

iii) On the basis of ACS observations of the central region of the cluster, we detected the largest Blue Straggler Stars (BSS) population ever detected in a stellar system containing more than 300 candidates (Ferraro et al. 2006). By studying their positional properties we have found that the BSSs share the same radial distribution of the other less massive cluster stars. This evidence indicates that this stellar system is not fully relaxed even in the central region. A further confirmation of this result was provided by measuring the luminosity function of $\omega$ Cen at a different distance from the cluster centre (Sollima et al. 2007).

iv) Numerical models have been computed for the chemical evolution of the anomalous globular cluster $\omega$ Cen (see Sect. 1.1.1). In Romano et al. (2007), we have examined both the hypotheses that $\omega$ Cen be a real globular cluster or the relic nucleus of an accreted dwarf Speroidal galaxy, disrupted during the capture by the Milky Way. We have found that only in the latter scenario are models able to reproduce the observed chemical properties of the system. What remains unclear is how $\omega$ Cen managed to get the amazingly high helium abundance of $\rm Y=0.38$ suggested in the literature to explain the observed secondary blue main-sequence. Additional chemical evolution models are being computed by our group to better understand this point.

In summary, the Bologna key project on $\omega$ Cen is fully active, with a wealth of data being reduced and new observations being continuously performed. The work is in collaboration with scientists of several international institutes, among which F. Ferraro (Astr. Dept., Univ. of Bologna), O. Straniero (INAF-Teramo Obs.), M. Catelan (Un. Cat. Chile), J. Borissova (ESO), D. Minniti (Un. Cat. Chile), H. Smith (MSU), and R. T. Rood (UVA).