Spectroscopic Study on the Beryllium Abundances of Red Giant Stars

TL;DR

This study analyzes beryllium abundances in 200 red giant stars, revealing significant depletion likely caused by extra mixing processes beyond standard stellar evolution theories.

Contribution

It provides the first extensive spectroscopic analysis of Be in red giants, highlighting the role of non-standard mixing processes in surface Be depletion.

Findings

Be is depleted by 1-3 dex in red giants compared to initial levels.

Be depletion correlates weakly with stellar parameters but aligns with metallicity-scaled abundances.

Standard dredge-up theory cannot fully explain the observed Be deficits.

Abstract

An extensive spectroscopic study was carried out for the beryllium abundances of 200 red giants (mostly of late G and early K type), which were determined from the near-UV Be II 3131.066 line based on high-dispersion spectra obtained by Subaru/HDS, with an aim of investigating the nature of surface Be contents in these evolved giants; e.g., dependence upon stellar parameters, degree of peculiarity along with its origin and build-up timing. We found that Be is considerably deficient (to widely different degree from star to star) in the photosphere of these evolved giants by ~1-3 dex (or more) compared to the initial abundance. While the resulting Be abundances (A(Be)) appear to weakly depend upon T_eff, log g, [Fe/H], M, age, and v_sin i, this may be attributed to the metallicity dependence of A(Be) coupled with the mutual correlation between these stellar parameters, since such tendencies almost disappear in the metallicity-scaled Be abundance ([Be/Fe]). By comparing the Be abundances (as well as their correlations with Li and C) to the recent theoretical predictions based on sophisticated stellar evolution calculations, we concluded that such a considerable extent/diversity of Be deficit is difficult to explain only by the standard theory of first dredge-up in the envelope of red giants, and that some extra mixing process (such as rotational or thermohaline mixing) must be responsible, which presumably starts to operate already in the main-sequence phase. This view is supported by the fact that appreciable Be depletion is seen in less evolved intermediate-mass B-A type stars near to the main sequence.