Inelastic O+H collisions and the OI 777nm solar centre-to-limb variation

TL;DR

This study develops a physically-motivated model for inelastic O+H collisions that accurately reproduces the solar centre-to-limb variation of the OI 777 nm triplet, improving oxygen abundance measurements in stellar atmospheres.

Contribution

A new model for inelastic O+H collisional rates using an asymptotic two-electron approach, eliminating the need for calibration and enhancing non-LTE line formation accuracy.

Findings

Reproduces the solar centre-to-limb variation to 0.02 dex

Supports a low solar oxygen abundance of log epsilon_O=8.69

Improves the physical basis of non-LTE oxygen line modeling

Abstract

The OI 777 nm triplet is a key diagnostic of oxygen abundances in the atmospheres of FGK-type stars; however it is sensitive to departures from local thermodynamic equilibrium (LTE). The accuracy of non-LTE line formation calculations has hitherto been limited by errors in the inelastic O+H collisional rate coefficients: several recent studies have used the so-called Drawin recipe, albeit with a correction factor $\mathrm{S_{H}}$ that is calibrated to the solar centre-to-limb variation of the triplet. We present a new model oxygen atom that incorporates inelastic O+H collisional rate coefficients using an asymptotic two-electron model based on linear combinations of atomic orbitals, combined with a free electron model, based on the impulse approximation. Using a 3D hydrodynamic stagger model solar atmosphere and 3D non-LTE line formation calculations, we demonstrate that this physically-motivated approach is able to reproduce the solar centre-to-limb variation of the triplet to 0.02 dex, without any calibration of the inelastic collisional rate coefficients or other free parameters. We infer $\log\epsilon_{\mathrm{O}}=8.69\pm0.03$ from the triplet alone, strengthening the case for a low solar oxygen abundance.