Carbon cations and silicon atoms in the ISM: modelling their charge exchange reaction

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 interstellar medium 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 ultraviolet flux is very small, while becoming less important at higher T values and higher photon fluxes.