A NLTE line formation for neutral and singly-ionised calcium in model atmospheres of B-F stars

TL;DR

This paper develops NLTE line formation calculations for calcium in B-F stars, improving abundance accuracy and resolving discrepancies between different spectral lines, with applications to Gaia data and stellar diffusion models.

Contribution

It introduces a comprehensive NLTE method for calcium line formation in B-F stars, providing corrections applicable to Gaia spectra and stellar abundance studies.

Findings

NLTE abundances are consistent across Ca I and Ca II lines.

NLTE corrections resolve LTE discrepancies between visible and IR Ca II lines.

The method reproduces emission lines in a late B-type star.

Abstract

We present non-local thermodynamic equilibrium (NLTE) line formation calculations for Ca I and Ca II in B-F stars. The sign and the magnitude of NLTE abundance corrections depend on line and stellar parameters. We determine calcium abundances for nine stars with reliable stellar parameters. For all stars, where the lines of both species could be measured, the NLTE abundances are found to be consistent within the error bars. We obtain consistent NLTE abundances from Ca II lines in the visible and near infra-red (IR, 8912-27, 9890 A) spectrum range, in contrast with LTE, where the discrepancy between the two groups of lines ranges from -0.5 dex to 0.6 dex for different stars. Our NLTE method reproduces the Ca II 8912-27, 9890 A lines observed in emission in the late B-type star HD 160762 with the classical plane-parallel and LTE model atmosphere. NLTE abundance corrections for lines of Ca I and Ca II were calculated in a grid of model atmospheres with 7000 K < Teff < 13000 K, 3.2 < log g < 5.0, -0.5 < [Fe/H] < 0.5, Vt = 2.0 km/s. Our NLTE results can be applied for calcium NLTE abundance determination from Gaia spectra, given that accurate continuum normalisation and proper treatment of the hydrogen Paschen lines are provided. The NLTE method can be useful to refine calcium underabundances in Am stars and to provide accurate observational constraints on the models of diffusion.