Lithium Inventory of 2 $M_\odot$ Red Clump Stars in Open Clusters: A Test of the Helium Flash Mechanism

TL;DR

This study investigates lithium enrichment in 2 solar mass red clump stars within open clusters to test if the helium flash mechanism causes short-lived Li-rich phases, finding limited evidence for this process.

Contribution

It provides observational constraints on the helium flash hypothesis for Li enrichment, suggesting the process may be rare or long-lived rather than short-lived.

Findings

Few Li-rich stars observed, with only two possibly being RC stars.

No RC stars showed Li levels higher than RGB stars.

Low 12C/13C ratios suggest long-lived or rare Li enrichment events.

Abstract

The temperature distribution of field Li-rich red giants suggests the presence of a population of Li-rich red clump (RC) stars. One proposed explanation for this population is that all stars with masses near 2 $M_\odot$ experience a short-lived phase of Li-richness at the onset of core He-burning. Many of these stars have low 12C/13C, a signature of deep mixing that is presumably associated with the Li regeneration. To test this purported mechanism of Li enrichment, we measured abundances in 38 RC stars and 6 red giant branch (RGB) stars in four open clusters selected to have RC masses near 2 $M_\odot$. We find six Li-rich stars (A(Li) > 1.50 dex) of which only two may be RC stars. None of the RC stars have Li exceeding the levels observed in the RGB stars, but given the brevity of the suggested Li-rich phase and the modest sample size, it is probable that stars with larger Li-enrichments were missed simply by chance. However, we find very few stars in our sample with low 12C/13C. Such low 12C/13C, seen in many field Li-rich stars, should persist even after lithium has returned to normal low levels. Thus, if Li synthesis during the He flash occurs, it is a rare, but potentially long-lived occurrence rather than a short-lived phase for all stars. We estimate a conservative upper limit of the fraction of stars going through a Li-rich phase to be <47\%, based on stars that have low 12C/13C for their observed A(Li).