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

Abstract

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.

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

“…A MgAl anti-correlation has not been identified in NGC 6397, but Mg also seems to exhibit a certain scatter (Korn et al 2007). Lind et al (2009), hereafter Paper I, presented Na abundances for >100 TO, SGB, and RGB stars, and found that the most heavily Na-enriched stars are also significantly depleted in Li. It is thus clear that NGC 6397, like other globular clusters, should no longer be regarded as a single stellar population despite the tightness of its colour-magnitude diagram.…”

The lithium history of NGC 6397

“…Richard et al 2005), we find strong support for the need of some turbulence, with strict limits to its efficiency (cf. figure 10 in Lind et al 2009b). In absence of macroscopic turbulence, the turnoff and subgiant stars would deplete too much lithium compared to more unevolved stars, giving rise to large (∼0.6 dex) variations in surface-lithium abundance with evolutionary phase, which are not reproduced by observations.…”

“…For approximately one third of the sample we also covered the Na doublet at 8183Å and 8194Å, which is a fundamental ingredient to properly disentangle between in situ depletion and pre-enrichment effects (by testing the existence of a Li-Na anticorrelation, if any). More details about the observations and the reduction of the data set can be found in Lind et al (2009b).…”

Helium-rich stars in globular clusters: constraints for self-enrichment by massive stars

“…The first evidence rests on the chemical analysis that reveals large star-to-star abundance variations in light elements in all individual clusters studied so far, while the iron abundance stays constant (for a review see Gratton et al 2004). These variations include the well-documented anticorrelations between C-N, O-Na, Mg-Al, Li-Na and F-Na (Kraft 1994;Carretta et al 2007;Gratton et al 2007;Carretta et al 2006;Bonifacio et al 2007;Lind et al 2009). This global chemical pattern requires H-burning at high temperature around 75 × 10 6 K (Arnould et al 1999;Prantzos et al 2007).…”