Title: Effect of Matter Accretion on Lithium Enhancement of Giants Show Chinese title

Title: 物质吸积对巨星锂增强的影响

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. Show Chinese abstract

Abstract: 一些低质量巨星（$<2.2\,M_{\odot}$）表现出异常的锂增强行为，这仍然是一个开放性问题。 由于质量更大的巨星保留更多的表面锂，通过吸积周围物质增加质量可能是丰富锂的一种途径。 我们在当前的研究中评估了这一过程。 使用MESA，我们构建了一个包括质量损失的物质吸积模型，该模型使恒星从主序星转折点演化到红巨星分支顶端。 平均吸积率是根据吸积质量的上限和该期间恒星的演化时间估算的，并构建了一个吸积率网格。 我们分别考虑了吸积在质量和成分方面对巨星锂增强的影响。 具有较高锂丰度的吸积物质对巨星的锂增强有促进作用。 富含锂的吸积物质减轻了第一次翻涌过程中对流包层中锂的稀释。 增加的质量导致对流包层底部温度降低，从而同样削弱了表面锂的消耗。 弱的周围物质吸积可能是巨星锂增强的可能途径，并预测了锂丰度的上限为$\rm \sim 2.5\,dex$。 然而，它所需的质量增加对真实的天体物理环境提出了潜在的挑战。 这种吸积抑制了第一次翻涌期间恒星的锂稀释和消耗，因此表现出锂增强行为。