Excitation and charge transfer in low-energy hydrogen atom collisions with neutral carbon and nitrogen

A M Amarsi,P S Barklem

doi:10.1051/0004-6361/201935101

Abstract

Low-energy inelastic collisions with neutral hydrogen atoms are important processes in stellar atmospheres, and a persistent source of uncertainty in non-LTE modelling of stellar spectra. We have calculated and studied excitation and charge transfer of C I and of N I due to such collisions. We used a previously presented method that is based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions for the adiabatic potential energies, combined with the multichannel Landau-Zener model for the collision dynamics. We find that charge transfer processes typically lead to much larger rate coefficients than excitation processes do, consistent with studies of other atomic species. Two-electron processes were considered and lead to non-zero rate coefficients that can potentially impact statistical equilibrium calculations. However, they were included in the model in an approximate way, via an estimate for the two-electron coupling that was presented earlier in the literature: the validity of these data should be checked in a future work.

Highlights

Carbon and nitrogen, respectively the second and fourth most abundant metals in the Sun, are among the most important elements in modern astrophysics
We find that charge transfer processes typically lead to much larger rate coefficients than excitation processes do, consistent with studies of other atomic species
Based on earlier work where comparisons could be done with full quantum calculations, as well as indications from calculations using alternate couplings (Barklem 2016a), the uncertainties for the largest rates from these linear combination of atomic orbitals (LCAO) calculations are expected to be about one order of magnitude, with uncertainties becoming progressively larger as the rates become smaller

Summary

Introduction

Respectively the second and fourth most abundant metals in the Sun, are among the most important elements in modern astrophysics. They play a key role in stellar physics as catalysts in the CNO-cycle. As a result of this cycle their abundances are interlinked, and their abundances in the surfaces of stars that have dredged-up or mixed C-poor, N-rich material from the interior has been used to study stellar evolution (e.g. Gratton et al 2000; Spite et al 2005). Lines of C i and N i are commonly used to measure carbon and nitrogen abundances, in the Sun Lines of C i and N i are commonly used to measure carbon and nitrogen abundances, in the Sun (e.g. Asplund et al 2009; Caffau et al 2009, 2010) and early- and late-type stars (e.g. Tomkin & Lambert 1978; Clegg et al 1981; Takeda 1994; Przybilla et al 2001; Przybilla & Butler 2001; Shi et al 2002; Takeda & Honda 2005; Lyubimkov et al 2011, 2015)

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Conclusion