Evolution of lithium in the Milky Way halo, discs, and bulge

Abstract

In this work, we study the Galactic evolution of lithium by means of chemical evolution models in the light of the most recent spectroscopic data from Galactic stellar surveys. We consider detailed chemical evolution models for the Milky Way halo, discs and bulge, and we compare our model predictions with the most recent spectroscopic data for these different Galactic components. In particular, we focus on the decrease of lithium at high metallicity observed by the AMBRE Project, the Gaia-ESO Survey, and other spectroscopic surveys, which still remains unexplained by theoretical models. We analyse the various lithium producers and confirm that novae are the main source of lithium in the Galaxy, in agreement with other previous studies. Moreover, we show that, by assuming that the fraction of binary systems giving rise to novae is lower at higher metallicity, we can suggest a novel explanation to the lithium decline at supersolar metallicities: the above assumption is based on independent constraints on the nova system birthrate, that have been recently proposed in the literature. As regards to the thick disc, it is less lithium enhanced due to the shorter timescale of formation and higher star formation efficiency with respect to the thin disc and, therefore, we have a faster evolution and the "reverse knee" in the A(Li) vs. [Fe/H] relation is shifted towards higher metallicities. Finally, we present our predictions about lithium evolution in the Galactic bulge, that, however, still need further data to be confirmed or disproved.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…The two bulge stars with the highest Li abundances at [Fe/H] ≈ +0.4 have A(Li) ≈ 2.8, and seem to continue on the declining A(Li) trend with [Fe/H] as outlined by the disk stars. Grisoni et al (2019) modelled the evolution of Li in the Milky Way and concluded that the main producer of Galactic Li is novae. By assuming that the fraction of binaries, which are supposed to be the novae progenitors, decrease with increasing metallicity, they also could provide an explanation for the decrease in A(Li) seen at the very highest metallicities.…”

“…The thin dash-dotted lines in Fig. 6 shows the models from Grisoni et al (2019) in which Li is produced by novae. There are several novae models shown, representing different laws for the fraction of binary systems that give rise to novae (see Grisoni et al 2019 for more details).…”

“…6 shows the models from Grisoni et al (2019) in which Li is produced by novae. There are several novae models shown, representing different laws for the fraction of binary systems that give rise to novae (see Grisoni et al 2019 for more details). The solid thick line shows the total production of Li in the disk and includes other sources that are less significant, such as AGB stars, cosmic rays, and massive stars.…”

“…The solid thick line shows the total production of Li in the disk and includes other sources that are less significant, such as AGB stars, cosmic rays, and massive stars. The evolution of Li in the thick disk is shown by the thick dashed line, and the rise at solar metallicities is caused by Li production in novae that starts to contribute to the Galactic Li enrichment about one billion years after the beginning of Galaxy formation (Grisoni et al 2019). The thin dashed line shows the model for the Galactic bulge, which Grisoni et al (2019) assumes to be a classical bulge population, meaning no young stars, only an old classical spheroid.…”

“…Another possibility could be that the apparent decline in the Li trend in the solar neighbourhood is due to stars that have migrated here from the inner disk region, or even the bulge region, and that they show a lower Li abundance due to a faster star formation history in the inner disk (or bulge) regions (Guiglion et al 2019). Grisoni et al (2019) provide a new explanation, in which novae are the main Li producers and the decline at the highest metallicities could be due to a metallicity-dependent fraction of the binary systems that are novae precursors. No satisfactory explanation has so far been provided as to whether the migration scenario or the metallicity-dependent binary fraction scenario is preferred.…”