Modeling of the SiO Circumstellar Maser: Rate Coefficients for the Vibrationally‐Rotationally Inelastic Scattering of H+SiO

Abstract

Rate coefficients for inelastic collisions of H atoms with SiO molecules are presented over a wide range of temperatures. These coefficients are needed in the modeling of the observed SiO maser radiation that originates from circumstellar environments. The rate coefficients have been computed theoretically. They are based initially on ab initio calculation of the H-SiO interaction potential. This is followed by an approximate quantum scattering calculation, using the vibrationally close-coupling infinite-order sudden method, of the vibrationally-rotationally inelastic scattering cross sections over a relatively large range of scattering energies. The rate coefficients presented here are finally computed by suitably averaging the cross sections over an equilibrium Boltzmann distribution of relative translational energies. The paper discusses the general form and properties of the rate constants and provides a computer code to compute any desired vibrational-rotational inelastic rate constant for vibrational quantum number v = 0 to 5 and rotational quantum number j = 0 to 40. The computer code will return rate constants for any desired temperature, and the computed rate constants should be reliable in the important temperature range 100-5500 K. The quantum mechanically computed cross section data extend up to a temperature of 9600 K, which means that computed rate constants above about 5500 K become progressively less reliable with increasing temperature, as they depend to a greater extent on extrapolated data.