An empirical recipe for inelastic hydrogen-atom collisions in non-LTE calculations

TL;DR

This paper introduces an empirical method to estimate inelastic hydrogen collision rates in NLTE spectral line synthesis, matching quantum calculations with high accuracy across various stellar types and elements.

Contribution

The authors present a new polynomial fitting recipe for hydrogen collision rates that accurately reproduces quantum NLTE abundance corrections, improving modeling efficiency and applicability.

Findings

The method reproduces quantum NLTE corrections within 0.05 dex for NaI and AlI lines.

It performs better for cool giants and dwarfs than for subgiants and warm dwarfs.

Discrepancies with Drawin rates can be up to 0.4 dex, highlighting the importance of including CT and ionization processes.

Abstract

We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, NaI, MgI, AlI, SiI and CaI) using simple polynomial equations. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We then compare the results to calculations computed using the published quantum rates for all the corresponding elements. We also compare to results computed using excitation collisional rates via the commonly used Drawin equation for different fudge factors, SH, applied. We demonstrate that our proposed method is able to reproduce the NLTE abundance corrections performed with the quantum rates for different spectral types and metallicities for representative NaI and AlI lines to within $\le$0.05 dex and %\le%0.03 dex, respectively. For MgI and SiI lines, the method performs better for the cool giants and dwarfs, while larger discrepancies up to 0.2 dex could be obtained for some lines for the subgiants and warm dwarfs. We obtained larger NLTE correction differences between models incorporating Drawin rates relative to the quantum models by up to 0.4 dex. These discrepancies are potentially due to ignoring either or both CT and ionization collisional processes by hydrogen in our Drawin models. Our empirical fitting method performs well in its ability to reproduce, within narrow uncertainties, the abundance corrections computed with models incorporating quantum collisional rates. It could possibly be extended to other transitions or in the absence of published quantum calculations, to other elements as well.