The Gaia-ESO Survey: Galactic evolution of lithium at high metallicity

Abstract

Context. Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. Aims. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. Methods. We trace the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. Results. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] = ∼0.3) actually show the highest Li abundances, with A(Li) > 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it.

Light Elements in the Universe

“…Their results are reported in Figure 10; specifically, they very clearly demonstrated that previous claims of a Li decline at supersolar metallicities were due to selection biases in the used samples and that Li abundance in the ISM instead does not decrease at high metallicity but may actually increase. The data presented by Randich et al (2020) also suggested-for the first time-the presence of a mild trend of A(Li) with Galactocentric distance, or a shallow gradient, to be confirmed with the analysis of the full data set. Interestingly, in the last few years, Li observations in the Galactic thick disc and even Bulge have been reported (Delgado Mena et al, 2015;Bensby and Lind 2018;Fu et al, 2018;Bensby et al, 2020;Stonkutė et al, 2020).…”

“…Those latter suggestions were tested in a very recent paper by Randich et al (2020). They proposed the idea that Li measurements in metal-rich, young populations, that have presumably not depleted any Li and hence retain their original Li content, would allow tracing the evolution of Li in the ISM at high metallicity; to this aim, they exploited GES observations of open clusters and, specifically, of cluster members with supposedly pristine unprocessed Li, representative of the ISM value (very young PMS stars or stars on the warm side of the dip).…”

The behaviour of lithium at high metallicity in the Milky Way

“…These warm stars (T eff ≥ 6800 K) show relatively constant Li abundances close to the cosmic value, indicating that they have undergone no or minimal surface Li depletion. This has been observed in open clusters and field stars with metallicities close to solar where the Li abundance of stars on the hot side of the Li-dip can reach the meteoritic value (Hobbs & Pilachowski 1986a,b;Boesgaard & Tripicco 1986, 1987Boesgaard et al 1988;Burkhart & Coupry 1989;Pasquini et al 2001;Burkhart et al 2005;Anthony-Twarog et al 2021), and in more metal-rich open clusters where even higher Li values (A(Li) > 3.4) have been derived in the same effective temperature domain (Randich et al 2020). Some exceptions deviate from the mean, namely AmFm-type stars (Burkhart et al 2005;Burkhart & Coupry 1989 which are slow rotators that present Li deficiency (by about a factor of 3) compared to normal early A and late F type stars as well as other abundance anomalies due to atomic diffusion (gravitational settling and radiative levitation; discussion in Sect.…”

“…1), and they always exhibit the highest Li abundances in 1 Bensby et al (2020) focus on the Galactic bulge and have only 3 stars with well-determined Li, log g 4 and −0.2 < Fe/H 0.4, and they are all cool G-type stars (T eff = 6130 K, 5732 K, and 5947 K). Randich et al (2020) for which the authors selected upper MS stars on the warm side of the Li dip (above 6500 K actually). Their Li abundances were not corrected for NLTE effects, but the authors argue that these corrections should be significantly lower than +0.1 dex.…”

“…A few ideas were put forward to cope with this unprecedented situation (Fu et al 2018;Guiglion et al 2019;Grisoni et al 2019), although Prantzos et al (2017) had warned that the results deduced from AMBRE would require further analysis and additional observations before establishing such far reaching conclusions. This finding has indeed been challenged recently, by observations of warm MS stars in metal-rich open clusters (Gaia-ESO survey, Randich et al 2020) and in the field (GALAH survey, Gao et al 2020), which show high Li abundances above the meteoritic value, in line with Galactic chemical evolution expectations.…”