[Paper Review] A NLTE study of neutral boron in solar-type stars

D. Kiselman|arXiv (Cornell University)|Jan 12, 1994

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper presents a non-local thermodynamic equilibrium (NLTE) analysis of neutral boron lines in solar-type stars, showing that NLTE effects—driven by overionization and optical pumping—cause significant underestimation of boron abundance in LTE analyses. For HD140283 and Procyon, NLTE corrections are +0.6 dex and +0.4 dex respectively, while the Sun shows no significant NLTE effect, resolving discrepancies in beryllium-to-boron ratios in metal-poor stars.

ABSTRACT

The formation of the resonance lines of neutral boron in solar-type stellar atmospheres is investigated taking into account effects of departures from local thermodynamic equilibrium (NLTE effects). The latter are due to a combination of overionisation and optical pumping in resonance lines, both caused by the hot, non-local, ultraviolet radiation fields in the line-forming regions. They lead to an underestimation of the boron abundance when analysis methods assuming local thermodynamic equilibrium (LTE) are used. The abundance correction, for the 249.7~nm resonance line, amounts to $+$0.6 dex for the metal-poor star HD140283 and $+$0.4 dex for Procyon. No significant NLTE effects are predicted for the Sun. Applying the abundance correction on the results for HD140283 of Duncan et al. (1992) leads to a B/Be ratio well above the minimum value required by spallation production of beryllium. The reliability of the results in view of atomic and atmospheric uncertainties is discussed. With the possible exception of photospheric inhomogeneity, it seems unlikely that these could remove the effect for HD140283.

Motivation & Objective

• To investigate the impact of non-local thermodynamic equilibrium (NLTE) effects on the formation of neutral boron lines in solar-type stars.
• To quantify the magnitude of NLTE corrections for boron abundance determinations in stars with varying metallicity.
• To assess whether NLTE effects can explain the observed high B/Be ratios in metal-poor stars like HD140283.
• To evaluate the reliability of NLTE results in light of atomic and atmospheric uncertainties.
• To determine whether photospheric inhomogeneity or other factors could otherwise explain the observed abundance anomalies.

Proposed method

• A detailed NLTE radiative transfer model was constructed for neutral boron (B I) lines in stellar atmospheres.
• The model accounts for overionization and optical pumping by non-local ultraviolet radiation fields in line-forming regions.
• Radiative transfer equations were solved in the non-LTE regime using the escape probability method and statistical equilibrium equations.
• The 249.7 nm resonance line of neutral boron was analyzed in three stars: the Sun, Procyon, and the metal-poor star HD140283.
• NLTE abundance corrections were derived by comparing NLTE and LTE line strengths for each star.
• The results were cross-checked against observed spectra and previous LTE analyses, particularly for HD140283.

Experimental results

Research questions

• RQ1 How do NLTE effects influence the formation of the 249.7 nm resonance line of neutral boron in solar-type stars?
• RQ2 What is the magnitude of the NLTE abundance correction for boron in metal-poor stars like HD140283?
• RQ3 Why does the B/Be ratio in HD140283 exceed the minimum value expected from spallation production, and can NLTE effects explain this?
• RQ4 Are the NLTE corrections significant for the Sun, or are they negligible in solar-type stars with higher metallicity?
• RQ5 Could photospheric inhomogeneity or other atmospheric uncertainties account for the observed abundance anomalies instead of NLTE effects?

Key findings

• The NLTE abundance correction for the metal-poor star HD140283 is +0.6 dex, significantly increasing its derived boron abundance compared to LTE.
• For Procyon, the NLTE correction is +0.4 dex, indicating substantial non-LTE effects in this solar-type star.
• No significant NLTE effects are predicted for the Sun, suggesting NLTE corrections are negligible in solar-metallicity stars.
• Applying the NLTE correction to the HD140283 results from Duncan et al. (1992) leads to a B/Be ratio well above the minimum value expected from spallation nucleosynthesis.
• The study finds it unlikely that photospheric inhomogeneity or other uncertainties could explain the observed abundance anomaly in HD140283, supporting NLTE as the primary cause.
• The results demonstrate that NLTE effects are critical for accurate boron abundance determinations in metal-poor stars, especially when interpreting B/Be ratios.