Rate constants for the formation of SiO by radiative association

TL;DR

This paper calculates accurate rate constants for SiO formation via radiative association, considering both direct and indirect processes, and evaluates their relevance in different astrophysical environments.

Contribution

It introduces a comprehensive method to compute SiO formation rate constants including quasi-bound states and evaluates two limiting astrophysical scenarios.

Findings

Rate constants vary significantly between the two limiting models.

Indirect formation processes can dominate under certain conditions.

Results provide bounds for SiO formation rates in astrophysical environments.

Abstract

Accurate molecular data for the low-lying states of SiO are computed and used to calculate rate constants for radiative association of Si and O. Einstein A-coefficients are also calculated for transitions between all of the bound and quasi-bound levels for each molecular state. The radiative widths are used together with elastic tunneling widths to define effective radiative association rate constants which include both direct and indirect (inverse predissociation) formation processes. The indirect process is evaluated for two kinetic models which represent limiting cases for astrophysical environments. The first case scenario assumes an equilibrium distribution of quasi-bound states and would be applicable whenever collisional and/or radiative excitation mechanisms are able to maintain the population. The second case scenario assumes that no excitation mechanisms are available which corresponds to the limit of zero radiation temperature and zero atomic density. Rate constants for SiO formation in realistic astrophysical environments would presumably lie between these two limiting cases.