A NLTE analysis of boron abundances in metal-poor stars

TL;DR

This study investigates the NLTE line formation of boron in metal-poor stars, showing that NLTE effects are significant but smaller than previously thought, and applies this to determine boron abundances, revealing its primary production in the early Galaxy.

Contribution

It provides a revised analysis of NLTE effects on boron lines, showing closer to LTE conditions with increased hydrogen collisions, and applies this to stellar spectra to understand boron production.

Findings

Boron increases with oxygen in metal-poor stars.

B/Be ratio remains nearly constant across metallicities.

Boron production aligns with primary processes in early Galaxy.

Abstract

The non-local thermodynamic equilibrium (NLTE) line formation of neutral boron in the atmospheres of cool stars are investigated. Our results confirm that NLTE effects for the B I resonance lines, which are due to a combination of overionization and optical pumping effects, are most important for hot, metalpoor, and low-gravity stars; however, the amplitude of departures from LTE found by this work are smaller than that of previous studies. In addition, our calculation shows that the line formation of B I will get closer to LTE if the strength of collisions with neutral hydrogen increases, which is contrary to the result of previous studies. The NLTE line formation results are applied to the determination of boron abundances for a sample of 16 metal-poor stars with the method of spectrum synthesis of the B I 2497 A resonance lines using the archived HST/GHRS spectra. Beryllium and oxygen abundances are also determined for these stars. The abundances of the nine stars which are not depleted in Be or B show that, no matter the strength of collisions with neutral hydrogen may be, both Be and B increase with O quasi-linearly in the logarithmic plane, which confirms the conclusions that Be and B are mainly produced by primary process in the early Galaxy. The most noteworthy result of this work is that B increases with Fe or O at a very similar speed as, or a bit faster than Be does, which is in accord with the theoretical models. The B/Be ratios remain almost constant over the metallicity range investigated here. Our average B/Be ratio falls in the interval [13+-4, 17+-4], which is consistent with the predictions of spallation process. The contribution of B from the nu-process may be required if the 11B/10B isotopic ratios in metal-poor stars are the same as the meteoric value.