Constraining Extra Mixing during the Main Sequence: What Depletes Lithium Does Not Touch Beryllium

TL;DR

This study investigates lithium and beryllium depletion in solar-type stars, finding that beryllium remains largely undepleted with age, which constrains stellar mixing models and suggests early Be depletion within 1 Gyr.

Contribution

The paper provides new observational constraints on stellar mixing models by analyzing lithium and beryllium abundances in solar twins, showing Be does not deplete with age unlike Li.

Findings

Beryllium does not decrease with stellar age, unlike lithium.

Models with convective overshoot and settling are favored over rotational mixing.

Be depletion occurs within the first ~1 Gyr, and Be abundance increases with metallicity.

Abstract

Measurements of lithium abundances in solar-type stars have shown that standard models of stellar evolution are incapable of explaining the observed depletion as a function of stellar age. Beryllium is one of the lightest elements that can be measured in stellar photospheres, and it can be burned in relatively low temperatures. Studying its abundances as a function of stellar age can provide important constraints to stellar mixing models, as the level of depletion as a function of time will indicate how deep the photospheric material must be dredged to explain the observed abundances. In an effort to provide the most stringent constraints for non-standard stellar mixing models, we observed a sample of solar-twins and concomitantly analyzed their lithium and beryllium abundances. Unlike what is typically observed for lithium, we found that beryllium does not decrease as a function of stellar age along the main-sequence, constraining models that predict burning of both materials. Based on our data, models that invoke convective overshoot and convective settling are preferred over typical rotationaly-induced mixing models, as the later burn Be in excess while the former do not. Previous works also proposed mixing due to gravity waves as a possible explanation for observed abundances, which can fit our data as well. Furthermore, based on our solar twins, Be depletion likely happens within the first $\sim1$ Gyr. We also confirm previous findings of an increase in Be abundance as a function of metallicity, indicative of galactic production via cosmic ray spallation.