The lithium content of the Galactic Halo stars

Abstract

Thanks to the accurate determination of the baryon density of the universe by the recent cosmic microwave background experiments, updated predictions of the standard model of Big Bang nucleosynthesis now yield the initial abundance of the primordial light elements with unprecedented precision. In the case of 7 Li, the CMB+SBBN value is significantly higher than the generally reported abundances for Pop II stars along the so-called Spite plateau. In view of the crucial importance of this disagreement, which has cosmological, galactic and stellar implications, we decided to tackle the most critical issues of the problem by revisiting a large sample of literature Li data in halo stars that we assembled following some strict selection criteria on the quality of the original analyses. In the first part of the paper we focus on the systematic uncertainties affecting the determination of the Li abundances, one of our main goal being to look for the "highest observational accuracy achievable" for one of the largest sets of Li abundances ever assembled. We explore in great detail the temperature scale issue with a special emphasis on reddening. We derive four sets of effective temperatures by applying the same colour-T eff calibration but making four different assumptions about reddening and determine the LTE lithium values for each of them. We compute the NLTE corrections and apply them to the LTE lithium abundances. We then focus on our "best" (i.e. most consistent) set of temperatures in order to discuss the inferred mean Li value and dispersion in several T eff and metallicity intervals. The resulting mean Li values along the plateau for [Fe/H] ≤ -1.5 are A(Li) NLTE = 2.214 ± 0.093 and 2.224 ± 0.075 when the lowest effective temperature considered is taken equal to 5700 K and 6000 K respectively. This is a factor of ∼2.48 to 2.81 (depending on the adopted SBBN model and on the effective temperature range chosen to delimit the plateau) lower than the CMB+SBBN determination. We find no evidence of intrinsic dispersion. Assuming the correctness of the CMB+SBBN prediction, we are then left with the conclusion that the Li abundance along the plateau is not the pristine one, but that halo stars have undergone surface depletion during their evolution. In the second part of the paper we further dissect our sample in search of new constraints on Li depletion in halo stars. By means of the Hipparcos parallaxes, we derive the evolutionary status of each of our sample stars, and re-discuss our derived Li abundances. A very surprising result emerges for the first time from this examination. Namely, the mean Li value as well as the dispersion appear to be lower (although fully compatible within the errors) for the dwarfs than for the turnoff and subgiant stars. For our most homogeneous dwarfs-only sample with [Fe/H] ≤ -1.5, the mean Li abundances are A(L) NLTE = 2.177 ± 0.071 and 2.215 ± 0.074 when the lowest effective temperature considered is taken equal to 5700 K and 6000 K respectively. This is a factor of 2.52 to 3.06...

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…In our warm (T eff  5800 K) stars, the mean is log ε Li = 2.2, which is consistent with log ε Li = 2.177±0.071 deduced by Charbonnel & Primas (2005) for the [Fe/H] −1.5 stars with T eff  5700 K. Further theoretical studies of stellar physics and evolution are needed to understand source(s) of discrepancy with the standard BBN that predicts log ε Li = 2.72±0.06 (Coc et al 2012) and discrepancy between the warm and cool (T eff <5800 K) halo dwarf stars.…”

“…Careful reanalysis of the literature data led Charbonnel & Primas (2005) to deduce log ε Li = 2.177±0.071 for the [Fe/H] −1.5 stars with T eff  5700 K. With the baryon-to-photon ratio defined accurately by the Wilkinson Microwave Anisotropy Probe, standard big bang nucleosynthesis (BBN) predicts a primordial lithium abundance of log ε Li = 2.64±0.03 (Spergel et al 2007) to 2.72±0.06 (Coc et al 2012). Several physical mechanisms were proposed to reduce the Li abundance at the surface of halo stars compared with the pristine one (see, e.g., Charbonnel & Primas 2005;Korn et al 2006;Fu et al 2015); however, the theoretical models see considerable difficulties in reconciling a non-negligible depletion of lithium with both the flatness and the small dispersion along the Spite plateau. It is worth noting that the NLTE corrections for the only line, Li I λ6707, used in the abundance determinations are mostly negative and small in absolute value (Figure 2).…”

The lithium history of NGC 6397

“…Figure 3 illustrates how the mean lithium abundance varies with visual magnitude in the cluster and also compares our findings to other, similar studies of NGC 6397. A feature seen in all three of the most recent studies is that subgiants appear more lithium-rich than less evolved stars, as already suggested by Charbonnel & Primas (2005) for field stars. Our observations reveal a small upturn in Li abundance, located just prior to the stage where Li is strongly diluted, which may indicate that an overabundance is formed above Li-free layers in the stellar interior.…”

The metal-poor end of the Spite plateau

“…15: the Asplund et al (2006) sample is biased, having been purposedly selected not to include objects significantly deviating from the Spite plateau. In fact, stars with varying degrees of Li depletion have long been known to exist (e.g., Charbonnel & Primas 2005). On the other hand, warm (i.e., T eff > 6000 K) dwarf stars deviating from the Spite Plateau appear to be quite rare at [Fe/H] > −3.…”