Chemical abundances in the multiple sub-giant branch of 47 Tucanae: insights on its faint sub-giant branch component

Abstract

The globular cluster 47 Tuc exhibits a complex sub-giant branch (SGB) with a faint-SGB comprising only about the 10% of the cluster mass and a bright-SGB hosting at least two distinct populations. We present a spectroscopic analysis of 62 SGB stars including 21 faint-SGB stars. We thus provide the first chemical analysis of the intriguing faint-SGB population and compare its abundances with those of the dominant populations. We have inferred abundances of Fe, representative light elements C, N, Na, and Al, α elements Mg and Si for individual stars. Oxygen has been obtained by co-adding spectra of stars on different sequences. In addition, we have analysed 12 stars along the two main RGBs of 47 Tuc.Our principal results are: (i) star-to-star variations in C/N/Na among RGB and bright-SGB stars; (ii) substantial N and Na enhancements for the minor population corresponding to the faint-SGB; (iii) no high enrichment in C+N+O for faint-SGB stars. Specifically, the C+N+O of the faint-SGB is a factor of 1.1 higher than the bright-SGB, which, considering random (±1.3) plus systematic errors (±0.3), means that their C+N+O is consistent within observational uncertainties. However, a small C+N+O enrichment for the faint-SGB, similar to what predicted on theoretical ground, cannot be excluded. The N and Na enrichment of the faint-SGB qualitatively agrees with this population possibly being He-enhanced, as suggested by theory. The iron abundance of the bright and faint-SGB is the same to a level of ∼0.10 dex, and no other significant difference for the analysed elements has been detected.

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…We note also that an additional paucity of stars may be present near [Na/Fe] ∼ 0.35 dex, which may further distinguish the most Na/Al-rich stars. These stars, which constitute ∼33% of our sample, are likely equivalent to the "extreme" population found by Carretta et al (2009c) in several clusters, the "E" population of NGC 2808 (Milone et al 2015b), and the faint sub-giant branch stars of 47 Tuc (Marino et al 2016). We note that a similarly high fraction of very Na/Al-rich stars is also found in ω Cen (Johnson & Pilachowski 2010;Marino et al 2011a) and M54 ).…”

“…A key observational constraint for globular cluster formation scenarios is whether the range of light element abundances follows a continuous distribution or falls into discrete groups. Although some purely spectroscopic evidence supports clusters hosting discrete groups with unique light element chemistry (e.g., Carretta et al 2009cCarretta et al , 2014Johnson & Pilachowski 2010;Carretta 2014Carretta , 2015Cordero et al 2014;Roederer & Thompson 2015), HST photometry has been particularly efficient at showing that most or all Galactic globular clusters host multiple distinct populations rather than continuous distributions (e.g., Piotto et al 2007Piotto et al , 2015Bragaglia et al 2010b;Milone et al 2013Milone et al , 2015aMilone et al , 2015bMarino et al 2016). The combined data from spectroscopy and photometry provide strong evidence that globular clusters experienced multiple rounds of star formation.…”

Revisiting nucleosynthesis in globular clusters

“…This is supported by the fact that stars on the different sequences also exhibit different chemical compositions (e.g. Marino et al 2008Marino et al , 2016Yong et al 2008;Milone et al 2012c). Indeed, multi-object spectroscopy with VLT has extensively documented the chemical properties of MSPs, revealing that GC stars located in all the regions of the CMD (main sequence, turnoff, red giant, horizontal and asymptotic giant branches) exhibit large abundance variations of C, N, O, Na, Al, and sometimes Mg that are coupled with anticorrelations (C-N, Na-O, Mg-Al, Li-Na; e.g.…”

TheGaia-ESO Survey: Galactic evolution of sulphur and zinc

“…ges_S_Zn_arXiv Jacobson et al (2016) and Spina et al (2017), due to the use of different solar reference values, and to the inclusion only of the subset of stars with measured sulphur. Marino et al 2016). It is particularly intriguing that when stars belonging to the bright and faint SGB of 47 Tuc (bSGB, fSGB) are discriminated in Marino et al (2016), the bSGB stars appear Na-poor, and match the range in Na observed by Dobrovolskas et al (2014) and Sbordone et al (2009).…”