Beryllium abundances and the formation of the halo and the thick disk

TL;DR

This paper investigates beryllium abundances in stars to understand the formation history of the Galactic halo and thick disk, revealing distinct stellar populations and formation processes through chemical analysis.

Contribution

It provides new evidence of two stellar components in the halo and insights into the inside-out formation of the thick disk based on beryllium and alpha-element data.

Findings

Halo stars separate into two components in Be vs. alpha/Fe diagram.

One component matches evolutionary model predictions.

Outer thick disk regions show lower star formation rates.

Abstract

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.