Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tucanae

Abstract

Context. The cluster 47 Tuc is among the most metal-rich Galactic globular clusters and its metallicity is similar to that of metal-poor disc stars and open clusters. Like other globular clusters, it displays variations in the abundances of elements lighter than Si, which is generally interpreted as evidence of the presence of multiple stellar populations. Aims. We aim to determine abundances of Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order to study the evolution of light elements in this cluster and to put our results in perspective with observations of other globular and open clusters, as well as with field stars. Methods. We use medium resolution spectra obtained with the GIRAFFE spectrograph at the ESO 8.2 m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employ CO 5 BOLD hydrodynamical simulations to assess the impact of stellar granulation on the line formation and inferred abundances. Results. Our results confirm the existence of Na-O abundance anti-correlation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. At the same time, we find no convincing evidence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were detected reliably, A(Li) 3D NLTE = 1.78 ± 0.18 dex, appears to be significantly lower than what is observed in other globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abundance observed in 47 Tuc is about 0.1 dex lower than the lowest Li abundance observed among the un-depleted stars of the metal-poor open cluster NGC 2243. Conclusions. The correlations/anti-correlations among light element abundances confirm that chemical enrichment history of 47 Tuc was similar to that of other globular clusters, despite the higher metallicity of 47 Tuc. The lithium abundances in 47 Tuc, when put into context with observations in other clusters and field stars, suggest that stars that are more metal-rich than [Fe/H] ∼ −1.0 experience significant lithium depletion during their lifetime on the main sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with which the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity.

Agb Sodium Abundances in the Globular Cluster 47 Tucanae (Ngc 104)

“…Instead, literature work suggests the cluster has an average [Fe/H] ≈ −0.60 to −0.80, with a star-to-star dispersion 0.10 dex (e.g., Carretta & Gratton 1997;AlvesBrito et al 2005;Wylie et al 2006;Koch & McWilliam 2008;Carretta et al 2009;Gratton et al 2013;Cordero et al 2014;Dobrovolskas et al 2014;Thygesen et al 2014). In agreement with past work, we find an average [Fe/H] = −0.68 and a starto-star dispersion of σ = 0.08.…”

“…We find an average heliocentric radial velocity of −18.56 km s −1 (σ = 10.21 km s −1 ). The average cluster velocity determined here is in good agreement with past work, which ranges from −16.86 to −22.43 km s −1 (Dinescu et al 1999;Carretta et al 2004;Alves-Brito et al 2005;Carretta et al 2009;Dobrovolskas et al 2014;Lapenna et al 2014). All of the stars observed with M2FS have velocities consistent with cluster membership.…”

“…While changes in [C/Fe], [N/Fe], and 12 C/ 13 C ratios as a function of evolutionary state on the subgiant branch (SGB) and red giant branch (RGB) are clearly linked to in situ mixing processes (e.g., Denissenkov & VandenBerg 2003), the temperatures reached near the bottom of the convective envelope in more evolved low-mass RGB stars are too low to significantly alter the abundances of heavier elements (but see also D'Antona & Ventura 2007 for a possible exception). Observations of similar star-to-star abundance variations among scarcely evolved globular cluster mainsequence and SGB stars (e.g., Briley et al 1996Briley et al , 2004Gratton et al 2001;Ramírez & Cohen 2002, 2003Carretta et al 2004;Cohen & Meléndez 2005;Bragaglia et al 2010;D'Orazi et al 2010;Dobrovolskas et al 2014) indicate that the composition differences between the various globular cluster populations were already imprinted on the gas from which the stars formed. Although there is still no consensus regarding the nucleosynthesis source(s) driving the composition differences, possible options include intermediate mass (∼5-8 M e ) asymptotic giant branch (AGB) stars (e.g., Karakas et al 2006;, rapidly rotating massive (20 M e ) main-sequence stars (e.g., Decressin et al 2007a), massive binary stars (de Mink et al 2009), and super massive (∼10 4 M e ) stars (Denissenkov & Hartwick 2014).…”

Nonthermal particles and photons in starburst regions and superbubbles

“…Anderson et al 2009). At least two different stellar populations were distinguished in 47 Tuc (NGC 104) by Dobrovolskas et al (2014) with small variation in the iron-peak and neutron capture elements but different abundances of proton-capture and alpha-elements. This suggests that a population of the cluster stars exists, which has most likely been formed from the gas enriched by the stars of the previous generation.…”

The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)

“…Other, independent studies concentrated A&A 578, A116 (2015) mostly on the closest GCs like 47 Tuc (e.g. Cordero et al 2014;Dobrovolskas et al 2014), M 4 (e.g. D'Orazi & Marino 2010D'Orazi et al 2013;Marino et al 2008), NGC 6397 (e.g.…”