The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Abstract

We analyze spectra of 18 stars belonging to the faintest subgiant branch in ω Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution of R 17 000 and a S /N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni, Si, and Ti and we find that these stars have [Fe/H] = −0.73 ± 0.14 dex, similarly to the corresponding red giant branch population (the RGB-a). We also measure [α/Fe] = +0.40 ± 0.16 dex, and [Ba/Fe] = +0.87 ± 0.23 dex, in general agreement with past studies. It is very interesting to note that we found a uniform Al abundance, [Al/Fe] = +0.32 ± 0.14 dex, for all the 18 SGB-a stars analysed here, thus supporting past evidence that the usual (anti-)correlations are not present in this population, and suggesting a non globular cluster-like origin of this particular population. In the dwarf galaxy hypothesis for the formation of ω Cen, this population might be the best candidate for the field population of its putative parent galaxy, although some of its properties appear contradictory. It has also been suggested that the most metal-rich population in ω Cen is significantly enriched in helium. If this is true, the traditional abundance analysis techniques, based on model atmospheres with normal helium content, might lead to errors. We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible. Additional, indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits.

C and N abundances of main sequence and subgiant branch stars in NGC 1851

“…M 08, Yong & Grundahl 2008), in close analogy to what was observed in M 22 and ω Centauri (e.g. Marino et al 2009Marino et al , 2011Johnson & Pilachowski 2010;Pancino et al 2011). In contrast, Carretta et al (2011) claimed that the faint-SGB consists of barium-poor metal-poor stars while bright-SGB stars have an enhanced barium and iron abundance.…”

FROM THE COLOR-MAGNITUDE DIAGRAM OF Ω CENTAURI AND (SUPER-)ASYMPTOTIC GIANT BRANCH STELLAR MODELS TO a GALACTIC PLANE PASSAGE GAS PURGING CHEMICAL EVOLUTION SCENARIO

“…Option (b) may also be favoured by observations of rather uniform Al abundances in the SGB-a population (in our scenario the third generation, see Table 5) by Pancino et al (2011), suggesting that no anti-correlation is present in this sub-population. Although we have tried to resist the temptation in this paper to apply the Galactic plane passage gas purging model to other clusters we note that Carretta et al (2012) found that the anticorrelations involving Al, Mg, Na and O in NGC 6752 -another GC for which multiple populations possibly including large He-enrichements have been found -manifest themselves in a rather discrete fashion, in which different levels of enhancement and depletion cluster around discrete values that can be associated with individual sub-populations, rather than a continuous distribution that would be sugggested by a pure dilution mechanism.…”

Stellar populations in ω Centauri: a multivariate analysis

“…SN II produce O and α-elements, but little Fe, while SN 1a produce mainly 56 Ni (which decays to form 56 Fe), and other isotopes of Ni in lower proportions, but not α-elements, so that [α/Fe] actually decreases (Iwamoto et al 1999, , their Table 3). SN 1a enrichment has been found in a few stars of the metal-rich "RGB-a" branch of ω Centauri (Pancino et al 2011). According to Marcolini et al (2007), SN 1a occur in small pockets of gas before being mixed with the interstellar medium, at variance with SN II, which pollute the interstellar medium uniformly.…”