# Tidal interactions between binary stars drives lithium production in low-mass red giants

## TL;DR

This study discovers a large sample of lithium-rich low-mass red giants and links their lithium enhancement to tidal interactions with binary companions, challenging previous explanations and revealing new insights into stellar evolution.

## Contribution

It provides the first large-scale evidence that tidal interactions in binary systems drive lithium production in red giants, with implications for stellar evolution models.

## Key findings

- Most lithium-rich giants have helium-burning cores.
- The frequency of lithium-rich giants increases with metallicity.
- Lithium-rich phase lasts about two million years.

## Abstract

Theoretical models of stellar evolution predict that most of the lithium inside a star is destroyed as the star becomes a red giant. However, observations reveal that about 1% of red giants are peculiarly rich in lithium, often exceeding the amount in the interstellar medium or predicted from the Big Bang. With only about 150 lithium-rich giants discovered in the past four decades, and no distinguishing properties other than lithium enhancement, the origin of lithium-rich giant stars is one of the oldest problems in stellar astrophysics. Here we report the discovery of 2,330 low-mass (1 to 3$\,M_\odot$) lithium-rich giant stars, which we argue are consistent with internal lithium production that is driven by tidal spin-up by a binary companion. Our sample reveals that most lithium-rich giants have helium-burning cores ($80^{+7}_{-6}\%$), and that the frequency of lithium-rich giants rises with increasing stellar metallicity. We find that while planet accretion may explain some lithium-rich giants, it cannot account for the majority that have helium-burning cores. We rule out most other proposed explanations as the primary mechanism for lithium-rich giants, including all stages related to single star evolution. Our analysis shows that giants remain lithium-rich for only about two million years. A prediction from this lithium depletion timescale is that most lithium-rich giants with a helium-burning core have a binary companion.