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

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).

16 pages, 7 Figures, accepted for publication in ApJ

References in corpus (5)

• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Precise radial velocities of giant stars. III. Spectroscopic stellar parameters
• Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
• Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment
• Rotational and Radial Velocities of 1.3-2.2 M_Sun Red Giants in Open Clusters

Cited by in corpus (6)

• Discovery of ubiquitous lithium production in low-mass stars
• Open Cluster Chemical Homogeneity Throughout the Milky Way
• Mass Matters: No Evidence for Ubiquitous Lithium Production in Low-Mass Clump Giants
• Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio
• Lithium in Kepler Red Giants: Defining Normal and Anomalous
• Chemical Compositions of Red Giant Stars in the Old Open Cluster NGC 7789