Rotational excitation of SiO by collisions with helium

Abstract

Context. Within shocked regions of the interstellar medium and circumstellar environment of AGB stars the proper modelling of SiO line emission through non-LTE radiative transfer calculations requires accurate values of collisional rate coefficients. Aims. The present study focuses on the transitions among the rotational levels of the SiO molecule in its ground vibrational state induced by collision with He. The H 2 molecule being the main colliding partner for the astrophysical regions of interest, the collisional process between SiO and para-H 2 (j = 0) is also investigated in an approximated way. Methods. A new 2D SiO-He potential energy surface is computed by means of highly correlated ab initio calculations. Collisional rate coefficients corresponding to the pure rotational (de)excitation of SiO by collision with He are obtained from close-coupling quantum scattering calculations of inelastic cross sections. The SiO-He potential energy surface is also employed to compute rate coefficients for the rotational (de)excitation of SiO by collision with para-H 2 (j = 0). Results. Rate coefficients for rotational levels up to j = 26 and kinetic temperatures in the range 10-300 K are obtained for the SiO-He colliding system. The large asymmetry of the SiO-He potential energy surface induces a propensity rule that favours odd Δj transitions over even Δj. The estimated values of the SiO-para-H 2 (j = 0) rate coefficients are compared with those of Turner et al. (1992) for the twenty first rotational levels. As a result of significant differences between the SiO-He interaction potentials employed in the two studies, the rate coefficients are found to differ by a factor 2.5-5 for the main rotational transitions, whatever the temperature range.