Correlated Depletion and Dilution of Lithium and Beryllium Revealed by Subgiants in M 67

TL;DR

This study investigates the depletion patterns of lithium and beryllium in subgiant stars of M 67, revealing correlated depletion due to stellar mixing processes, and compares observations with models including rotation-induced mixing.

Contribution

It provides the first detailed Be abundance measurements in M 67 subgiants and demonstrates their depletion patterns align with models of rotation-induced mixing, highlighting combined depletion and dilution effects.

Findings

Li drops by a factor of 400 across the subgiant branch.

Be declines by a factor of approximately 50.

Observed depletion patterns match models including stellar rotation effects.

Abstract

The surface content of lithium (Li) and beryllium (Be) provide insights into the mixing and circulation mechanisms in stellar interiors. The old open cluster, M 67, has been well-studied for Li abundances in both main-sequence and evolved stars. The Be abundances give us a probe to a deeper level in stars. We have taken high-resolution spectra with Keck I with HIRES to determine Be abundances along the subgiant branch of M 67, where there are dramatic depletions of Li. These subgiants range in mass from 1.26 to 1.32 M$_{\odot}$ and have evolved from main-sequence stars that would have occupied the region of the Li-Be dip found in younger clusters. Lithium abundances have been adjusted to the same scale for 103 stars in M 67 by Pace et al. The more massive stars $-$ now the coolest and furthest-evolved from the main-sequence $-$ show a drop in Li by a factor of 400 across the subgiant branch. Our new Be abundances also show a decline, but by a factor of $\sim$50. The two elements decline together with Li showing a steeper decline in these subgiants than it does in the Li-Be dip stars. The relative decline in Be abundance compared to Li is remarkably well fit by the models of Sills & Deliyannis, made specifically for the subgiants in M 67. Those models include the effects of mixing induced by stellar rotation. These M 67 subgiants show the effects of both main-sequence depletion and post-main-sequence dilution of both Li and Be.