The METALLICITY SPREAD AND THE AGE-METALLICITY RELATION OF Ω CENTAURI

Abstract

ω Centauri is a peculiar globular cluster formed by a complex stellar population. To investigate it, we studied 172 stars belonging to the five SGBs that we can identify in our photometry, in order to measure their [Fe/H] content as well as estimate their age dispersion and the age-metallicity relation. The first important result is that all of these SGBs have a distribution in metallicity with a spread that exceeds the observational errors and typically displays several peaks that indicate the presence of several subpopulations. We were able to identify at least six of them based on their mean [Fe/H] content. These metallicity-based subpopulations are seen to varying extents in each of the five SGBs. Taking advantage of the age sensitivity of the SGB, we showed that, first of all, at least half of the subpopulations have an age spread of at least 2 Gyr. Then, we obtained an age-metallicity relation that is the most complete to date for this cluster. Interpretation of the age-metallicity relation is not straightforward, but it is possible that the cluster (or what we can call its progenitor) was initially composed of two populations with different metallicities. Because of their age, it is very unlikely that the most metal-rich derives from the most metal-poor by some kind of chemical evolution process, so they can be assumed to be two independent primordial objects, or perhaps two separate parts of a single larger object, that merged in the past to form the present-day cluster.

The mosaic multiple stellar populations in ω Centauri: the horizontal branch and the main sequence

“…(vii) In conclusion, we show that a satisfactory compari-Authors Age ∆Age Ferraro et al (2004) 15 Gyr ∼ +2 Gyr Freyhammer et al (2005) 12 ÷ 13 Gyr ∼ +4 Gyr Lee et al (2005) 13 Gyr −1.5 Gyr Sollima et al (2005) 16.0 Gyr ∼ 0 ÷ 1 Gyr Stanford et al (2006) 13.5 Gyr −(2 ÷ 4) Gyr Joo & Lee (2013) 13.1 Gyr −1.7 Gyr Villanova et al (2014) 12.1 Gyr > ±2 Gyr This work 13 Gyr 0 ÷ −0.5 Gyr Table 3. Summary of isochrones fitting estimates of the maximum age difference among the populations in ω Centauri.…”

“…The left panel of Figure 1 shows about 50000 stars from the sample, chosen from the inner part of the cluster, with the aim to show the multiple MSs and SGBs observed. We over plot with green squares the location of the stars spectroscopically observed by Villanova et al (2014), hereinafter V14. Combining these two dataset, we identify three main sequences and five sub giant branches, as labelled in the figure.…”

“…A large number of populations that differ in light and heavy elements content have been spectroscopically identified, leading to classify ω Centauri as anomalous globular cluster (Johnson & Pilachowski 2010;Marino et al 2011;Villanova et al 2014, and references therein), alongside a few other (less extreme) clusters, such as M22 (Marino et al 2009), M2 (Yong et al 2014), and NGC 5286 (Marino et al 2015). Gratton et al (2011).…”

“…More recently, the latest published work fins this metal richest population 1.7 Gyr younger (Joo & Lee 2013). In addition, Villanova et al (2014) find huge age spreads within each population, by dating the subgiants with measured iron content. Now that more detailed information about the chemistry of the different populations is available (Johnson & Pilachowski 2010;Marino et al 2011Marino et al , 2012Villanova et al 2014) it is worth going back to the timescale of ω Centauri formation.…”

The HST large programme on ω Centauri – I. Multiple stellar populations at the bottom of the main sequence probed in NIR–Optical

“…This fact is quite expected. Indeed, it is well known from literature studies on the chromosome map of ω Centauri RGB stars that this cluster exhibit at least fifteen stellar populations (Milone et al 2017, see also Bellini et al 2010;Marino et al 2011;Villanova et al 2014). This suggests that each group of population A-D may host subpopulations of stars with different chemical composition that contribute to the ∆ 2 vs. ∆ 1 broadening of each population.…”