The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Abstract

We analysed high-resolution UVES spectra of six stars belonging to the subgiant branch of ω Centauri, and derived abundance ratios of 19 chemical elements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si, Sr, Ti, and Y). A comparison with previous abundance determinations for red giants provided remarkable agreement and allowed us to identify the sub-populations to which our targets belong. We found that three targets belong to a low-metallicity population at [Fe/H] -2.0 dex, [α/Fe] +0.4 dex and [s/Fe] 0 dex. Stars with similar characteristics were found in small amounts by past surveys of red giants. We discuss the possibility that they belong to a separate sub-population that we name VMP (very metal-poor, at most 5% of the total cluster population), which -in the self-enrichment hypothesis -is the best-candidate first stellar generation in ω Cen. Two of the remaining targets belong to the dominant metal-poor population (MP) at [Fe/H] -1.7 dex, and the last one to the metal-intermediate (MInt) one at [Fe/H] -1.2 dex. The existence of the newly defined VMP population could help to understand some puzzling results based on low-resolution spectroscopy for age differences determinations, because the metallicity resolution of these studies was probably not enough to detect the VMP population. The VMP could also correspond to some of the additional substructures of the subgiant-branch region found in the latest HST photometry. After trying to correlate chemical abundances with substructures in the subgiant branch of ω Cen, we found that the age difference between the VMP and MP populations should be small (0 ± 2 Gyr), while the difference between the MP and MInt populations could be slightly larger (2 ± 2 Gyr).

Manganese in dwarf spheroidal galaxies

“…3, but for Sculptor only (blue dots), showing a plateau followed by a decreasing trend rather than a monotonic decrease in [Mn/Fe]. Results for the globular cluster ω Centauri are also shown as red circles (Cunha et al 2010) and red dots (Pancino et al 2011 where Pearson's correlation coefficient is −0.569, Spearman's coefficient is −0.546, and the Student-t test is −4.51 (for 50 stars). This is clearly significant because the null hypothesis has a probability well below one percent.…”

“…The decreasing trend in [Mn/Fe] with increasing [Fe/H] seen in Sculptor had been observed nowhere else, except for giants and subgiants in the globular cluster ω Centauri (Cunha et al 2010;Pancino et al 2011), where the anti-correlation is even more pronounced (see Fig. 4).…”

Measuring helium abundance difference in giants of NGC 2808

“…Such a large difference in He abundance would increase the opacity, therefore modifying the spectra of these He-rich stars. This problem was studied by Bohm Vitense (1979), who pointed out that the effect is an increase in the gravity of the stars, as quantitatively addressed by Pancino et al (2011). A helium abundance increase by ΔY = 0.10 (from 0.25 to 0.35) in subgiant stars leads to a higher metallicity by Δ[Fe/H] = 0.08 dex, based on a simplified treatment of the atmosphere where the increased helium abundance is approximated by an artificial increase in the gravity.…”

The Pisa Stellar Evolution Data Base for low-mass stars

“…The situation is made additionally complex and interesting by an increasing number of discoveries within the most massive globular clusters of multiple stellar populations, photometrically distinct in the color-magnitude (CM) diagram (see e.g. Pancino et al 2011Pancino et al , 2000Carretta et al 2009;Villanova et al 2007;Norris 2004). Moreover, some of these populations seem to show a very high original helium abundance (up to Y ≈ 0.40) that is not accompanied by a corresponding increase in the iron abundance (see e.g.…”