[Paper Review] The NLTE formation of neutral-boron lines in cool stars

D. Kiselman, M. Carlsson|arXiv (Cornell University)|Jan 25, 1996

Stellar, planetary, and galactic studies3 references6 citations

TL;DR

This paper investigates the non-local thermodynamic equilibrium (NLTE) formation of neutral-boron (B I) lines in cool stellar atmospheres, showing that NLTE effects—primarily overionization and optical pumping—cause B I lines to be weaker than predicted by LTE, leading to underestimation of boron abundances if LTE is assumed. The authors provide NLTE abundance corrections for the B I 250 nm and 209 nm lines, demonstrating that the B/Fe ratio in metal-poor stars is approximately constant when NLTE effects are properly accounted for.

ABSTRACT

We study the formation of B I lines in a grid of cool stellar model atmospheres without the assumption of local thermodynamic equilibrium (LTE). The non-LTE modelling includes the effect of other lines blending with the B I resonance lines. Except for the cases where the B I lines are very strong, the departures from LTE relevant for the resonance lines can be described as an overionisation effect and an optical-pumping effect. This causes the lines to be weaker than in LTE so that an abundance analysis assuming LTE will underestimate stellar boron abundances. We present non-LTE abundance corrections useful to improve on abundances derived from the B I 250 nm and 209 nm lines under the LTE assumption. Application of the results on literature data indicates that the B/Fe ratio in metal-poor stars is constant.

Motivation & Objective

• To investigate the impact of non-LTE (NLTE) effects on the formation of neutral-boron (B I) lines in cool stellar atmospheres.
• To quantify how NLTE departures—specifically overionization and optical pumping—affect the strength of B I resonance lines.
• To derive NLTE abundance corrections for the B I 250 nm and 209 nm lines to improve accuracy in stellar boron abundance determinations.
• To re-analyze literature data on metal-poor stars using these NLTE corrections to assess the constancy of the B/Fe ratio.

Proposed method

• The study employs a grid of cool stellar model atmospheres with detailed radiative transfer calculations under NLTE conditions.
• NLTE calculations include the effects of line blending, particularly for the B I resonance lines at 250 nm and 209 nm.
• The modeling accounts for radiative transitions, ionization, and recombination processes beyond the LTE assumption.
• The researchers compute departure coefficients and line source functions to determine NLTE line strengths relative to LTE.
• NLTE abundance corrections are derived by comparing NLTE and LTE line strengths for various atmospheric parameters.
• The corrections are applied to existing literature data on metal-poor stars to reassess the B/Fe abundance ratio.

Experimental results

Research questions

• RQ1 How do NLTE effects alter the formation of neutral-boron (B I) lines in cool stellar atmospheres?
• RQ2 To what extent do overionization and optical pumping influence the strength of B I resonance lines?
• RQ3 What are the magnitude and dependence of NLTE abundance corrections for the B I 250 nm and 209 nm lines?
• RQ4 Does the B/Fe abundance ratio in metal-poor stars remain constant when NLTE effects are properly accounted for?

Key findings

• NLTE effects cause B I lines to be significantly weaker than predicted by LTE, primarily due to overionization and optical pumping.
• The NLTE abundance corrections for the B I 250 nm and 209 nm lines are substantial and necessary to avoid underestimating boron abundances.
• When NLTE corrections are applied, the B/Fe ratio in metal-poor stars is found to be approximately constant across different metallicities.
• The strength of B I lines in NLTE is reduced compared to LTE, especially in stars with low metallicity and high effective temperatures.
• The NLTE corrections are sensitive to atmospheric parameters such as temperature, gravity, and metallicity, but the overall trend of reduced line strength holds across the grid.
• The study provides publicly available computer routines for calculating NLTE abundance corrections, facilitating broader application in stellar abundance analysis.