Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 ,

By K. Lind, F. Primas, C. Charbonnel, F. Grundahl, Martin Asplund. 2009

203 citations recorded by the source corpus.

Context. To alleviate the discrepancy between the prediction of the primordial lithium abundance in the universe and the abundances observed in Pop II dwarfs and subgiant stars, it has been suggested that the stars observable today have undergone photospheric depletion of lithium. Aims. To identify the cause of this depletion, it is important to accurately establish the behaviour of lithium abundance with effective temperature and evolutionary phase. Stars in globular clusters are ideal objects for such an abundance analysis, because relative stellar parameters can be determined precisely. Methods. We conducted a homogeneous analysis of a very large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main sequence turn-off to high up on the red giant branch. Non-LTE Li abundances or abundance upper limits were obtained for all stars, and for a sizeable subset of the targets sodium abundances were also obtained. The Na abundances were used to distinguish stars formed out of pristine material from stars formed out of material affected by pollution from a previous generation of more massive stars. Results. The dwarf, turn-off, and early subgiant stars in our sample form a thin abundance plateau, disrupted in the middle of the subgiant branch by the Li dilution caused by the first dredge-up. A second steep abundance drop is seen at the luminosity of the red giant branch bump. The turn-off stars are more Li-poor, by up to 0.1 dex, than subgiants that have not yet undergone dredgeup. In addition, hotter dwarfs are slightly more Li-poor than cooler dwarfs, which may be a signature of the so-called Li dip in the cluster, commonly seen among Pop I stars. The feature is however weak. A considerably wide spread in Na abundance confirms that NGC 6397 has suffered from intracluster pollution in its infancy and a limited number of Na-enhanced and Li-deficient stars strongly contribute to forming a significant anti-correlation between the abundances of Na and Li. It is nevertheless seen that Li abundances are unaffected by relatively high degrees of pollution. Lithium abundance trends with effective temperature and stellar luminosity are compared to predictions from stellar structure models including atomic diffusion and ad-hoc turbulence below the convection zone. We confirm previous findings that some turbulence, with strict limits to its efficiency, is necessary for explaining the observations.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/0a565711f49988780935a0f533c8fba0b180ef48

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Nuclear and High Energy Physics

Citations in the corpus

THE MOST METAL-POOR STARS. IV. THE TWO POPULATIONS WITH [Fe/H] ≲ −3.0 (2012, background): “1, as the result of Li destruction in their convective envelopes (see, e.g., Lind et al. 2009, their Figure 5).”
7Li evolution in the thin and thick discs of the Milky Way (2018, background): “11 in Lind et al. (2009).”
N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes (2016, unknown): “…et al. (2015) LOS Watkins et al. (2015a) PM NGC 6397 Meylan & Mayor (1991) LOS Gebhardt et al. (1995) LOS Milone et al. (2006) LOS Carretta et al. (2009b) LOS Lind et al. (2009) LOS Lovisi et al. (2012) LOS Kamann et al. (2016) LOS Watkins et al. (2015a) PM NGC 6402 Kimmig et al. (2015)…”
Departures from LTE for neutral Li in late-type stars (2009, background): “Lithium depletion through atomic diffusion has been suggested as a solution to the discrepancy between the Spite plateau abundance and the predicted value of the primordial lithium abundance (see e.g. Korn et al. 2007; Lind et al. 2009).”
Photometric signatures of multiple stellar populations in galactic globular clusters (2011, background): “Lithium has been found to vary in a few cases (NGC 6752, NGC 6397, and 47 Tucanae, Pasquini et al. 2005; Bonifacio et al. 2007; Lind et al. 2009; Shen et al. 2010), anticorrelating with sodium, and hints exist that elements as heavy as sulfur might be affected (in 47 Tuc, Sbordone et al. 2009).”
THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, background): “At present, the observational tests have been limited to the moderately metal-poor globular clusters NGC 6397 (Korn et al. 2007; Lind et al. 2008, 2009b) and NGC 6752 (Korn 2010).”
Multiple populations in globular clusters (2012, background): “Lithium in a large sample of MS stars has been measured only in a few, nearby globular clusters: NGC 6397, where Lind et al. (2009) found a Li-Na anticorrelation; 47 Tuc, for which D’Orazi et al. (2010b) found no evidence of Li-O or Li-Na correlations; NGC 6752, where Shen et al. (2010) measured a…”
Properties of stellar generations in globular clusters and relations with global parameters (2010, background): “Seven of these GCs are actually thought to be 2 While writing this paper, Lind et al. (2009) published the analysis of an extended set of unevolved stars in NGC 6397, and we completed the analysis of turn-off stars in NGC 104 (D’Orazi et al. 2010), where Na abundances show similar variations.”
The SUMO project: a SUrvey of Multiple pOpulations in globular clusters (2013, background): “5 1990; Pasquini et al. 2004; Carretta et al. 2009), and a bimodality in sodium and oxygen (Lind et al. 2009, 2011).”
Abundance Estimates for 16 Elements in 6 Million Stars from LAMOST DR5 Low-Resolution Spectra (2019, background): “Even for a population with a single age and the same initial abundances, stars with different masses (and hence Teff ) can suffer different degrees of atomic diffusion (e.g. Korn et al. 2007; Lind et al. 2009; Önehag, Gustafsson & Korn 2014; Gao et al. 2018; Souto et al. 2019).”