CCD Photometry of the Globular Cluster Centauri. II. Stellar Populations and Age-Metallicity Relation

Abstract

We present wide-field and high-precision BV and Ca & Strömgren by photometry of ω Centauri, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of ω Cen are well discriminated from foreground Galactic field stars in the hk [=(Ca − b) − (b − y)] vs. b − y diagram. The resulting "cleaned" color-magnitude diagram (CMD) has allowed us to obtain an accurate distribution of the red horizontal branch (HB) and the asymptotic giant branch stars. We confirm the presence of several red giant branches (RGBs) with the most metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest that four populations with different metallicities can reproduce the observed nature of the RGB. The HB distribution is also found to be consistent with the multiple stellar populations of the RGB. From our population models, we propose that the most metal-rich population is about 4 Gyr younger than the dominant metal-poor population, indicating that ω Cen was enriched over this timescale. We identify, for the first time, a continuous and slanting RGB bump in the CMD of ω Cen, which is due to the metallicity spread amongst the RGB stars. Our photometry also reveals a significant population of blue straggler stars. The discovery of several populations and the internal age-metallicity relation of ω Cen provides good evidence that ω Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing at the present time.Subject headings: color-magnitude diagrams -globular clusters: generalglobular clusters: individual (ω Centauri) It also appears that this anomaly of chemical inhomogeneity found in ω Cen is closely linked to the kinematics and spatial distribution of the stars. Norris et al. (1997) found that the metal-rich RGB stars are more centrally concentrated and furthermore show evidence for different velocity dispersion and rotation properties compared to the dominating population of more metal-poor stars. The metal-poor population rotates, but the metal-rich stars do not. From this apparent difference in kinematics between the metal-poor and the metal-rich stars, as well as a second peak in the metallicity distribution,

Formation of multiple populations in globular clusters: another possible scenario

“…This is in contrast to early claims that ω Cen harbors stellar populations with an internal age spread that can reach several Gyr (e.g., Hughes & Wallerstein 2000;Rey et al 2004;Stanford et al 2006). We note, however, that these earlier results were obtained without taking into account the possibility of different levels of He enrichment in the different populations, which can lead to biases in the age estimates.…”

“…This GC has at least three separate, well-defined red giant branches (RGBs), an extended HB morphology, and a large number of subluminous extreme HB stars (e.g., D'Cruz et al 1996D'Cruz et al , 2000Whitney et al 1998;Pulone et al 1998;Pancino et al 2003;Rey et al 2004;Freyhammer et al 2005;Cassisi et al 2009;Bellini et al 2009;Calamida et al 2009Calamida et al , 2010. Hubble Space Telescope (HST) observations have also uncovered a triple MS (Bedin et al 2004;Bellini et al 2009Bellini et al , 2010, in addition to at least four SGBs (Villanova et al 2007).…”

Radial distribution of the multiple stellar populations in $\mathsf{\omega}$ Centauri

“…Our analysis confirms the results by Norris et al (1997), Hilker & Richtler (2000), Pancino et al (2000), Rey et al (2004), and Johnson et al (2009), who found that the most metalpoor RGB stars are less concentrated than the RGB-MInt ones. Moreover, we can also confirm that the RGB-a and the RGBMInt share the same radial distribution within ω Cen, as found by Norris et al (1997), Pancino et al (2000), and Pancino et al (2003) for RGB-a only.…”

“…Its stars cover a wide range in metallicity (Cannon & Stobie 1973;Norris & Bessell 1975, 1977Freeman & Rodgers 1975;Bessell & Norris 1976;Butler et al 1978;Norris & Da Costa 1995;Suntzeff & Kraft 1996;Norris et al 1996), with a primary component at [Fe/H] ∼ −1.7 to −1.8, and a long tail extending up to [Fe/H] ∼ −0.6, containing three or four secondary peaks (see Johnson et al 2009, for a recent update). It has been shown, both with ground-based photometry (Lee et al 1999;Pancino et al 2000;Rey et al 2004;Sollima et al 2005a;Villanova et al 2007) and Hubble Space Telescope (HST) photometry (Anderson 1997;Bedin et al 2004;Ferraro et al 2004), that ω Cen hosts different stellar populations, most of them clearly visible in most of their evolutionary phases.…”

“…On the other hand, the radial distribution of RGB subpopulations has been analyzed by many authors (Norris et al 1997;Hilker & Richtler 2000;Pancino et al 2000Pancino et al , 2003Rey et al 2004;Sollima et al 2005a;Castellani et al 2007;Johnson et al 2009). All these works agree that the intermediate-metallicity population (RGB-MInt) is more centrally concentrated than the more metal-poor one (RGB-MP).…”