Effect of Matter Accretion on Lithium Enhancement of Giants

Abstract

A subset of low-mass giants ($<2.2\,M_{\odot}$) exhibit anomalous lithium enhancement behavior, which is still an open topic. Given that more massive giants retain more surface lithium, increasing mass by accreting circumstellar matter could be a channel to enrich lithium. We evaluate this process in the current work. Using MESA, we construct a model of matter accretion, including mass loss, that evolves a star from the main sequence turnoff to the red giant branch tip. The mean accretion rate is estimated from the upper limit of the accreted mass and the evolutionary time of the star during this period, and a grid of accretion rates is constructed. We separately consider their effects on the lithium enhancement of giants, both in terms of the mass and the composition of accretion. Accreting matter with higher lithium abundances has a promoting effect on the lithium enhancement of giants. The accreted matter with excess lithium alleviates the dilution of lithium in the convective envelope during the first dredge-up. The added mass results in lower temperatures at the bottom of the convective envelope, which likewise weakens the depletion of surface lithium. Weak accretion of circumstellar matter is a possible route to lithium enhancement for giants, and it predicts an upper limit on the lithium abundance of $\rm \sim 2.5\,dex$. However, the mass increment it requires poses a potential challenge to real astrophysical environments. Such accretion suppresses lithium dilution and depletion of the star during the first dredge-up, thus exhibiting lithium enhancement behavior.