The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

TL;DR

This study assesses how uncertainties in electron collision data affect non-LTE lithium line formation in stellar atmospheres, finding minimal impact and confirming the reliability of existing data.

Contribution

It compares recent advanced electron collision calculations with previous data, demonstrating their agreement and negligible effect on non-LTE lithium abundance corrections.

Findings

Excellent agreement between CCC and RMPS calculations.

Small (~2%) differences in abundance corrections when using new vs. old data.

Uncertainty in electron collision data is negligible for non-LTE Li line formation.

Abstract

The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool stellar atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to recent calculations that use convergent close-coupling (CCC) techniques and to our own calculations using the R-matrix with pseudostates (RMPS) method. We find excellent agreement between rate coefficients from the CCC and RMPS calculations, and reasonable agreement between these data and the semi-empirical data used in non-LTE calculations up to now. The results of non-LTE calculations using the old and new data sets are compared and only small differences found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence on non-LTE calculations of uncertainties in the electron collision data is negligible. Indeed, together with the collision data for the charge exchange process Li(3s) + H <-> Li^+ + H^- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.