Carbon Cations and Silicon Atoms in the ISM: modeling their charge exchange reaction

TL;DR

This paper calculates new charge exchange reaction rates between carbon cations and silicon atoms in the interstellar medium using ab initio quantum methods, revealing significant differences from previous estimates especially in metal-rich, low-temperature environments.

Contribution

It provides the first ab initio derived rate coefficients for C+ + Si charge exchange reactions, improving the accuracy of ISM chemical models.

Findings

New rates differ significantly from empirical estimates in certain environments.

Differences are most notable in metal-rich, low-temperature conditions.

Results impact modeling of ISM chemistry and star formation processes.

Abstract

The time-dependent rate coefficients for the charge exchange reaction C+ + Si -> C + Si+ for doublet and quartet states have been determined with ab initio quantum calculations coupled with a non-adiabatic transition model based on a simple Landau-Zener picture. This reaction plays a key role in determining the abundances of C, Si, and their ions, in the ISM since these abundances affect the fine structure cooling and hence the star formation rates. We also provide additional calculations to evaluate the differences between the gas evolution as obtained by using the empirical rate estimates found in the current literature and the calculations presented in this work which are based on our more realistic evaluation of such rates from ab initio transition probabilities . We shall thus show here that the new rates yield important differences for metal-rich environments where $T<10^4$ K and the UV flux is almost negligible, while becoming less important at higher T values and higher photon fluxes.