Collisional depolarization of molecular lines. Application to the SiO+H isotropic collisions

Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain e-mail: moncef@iac.es

Received: 8 September 2005
Accepted: 2 November 2005

Abstract

This paper reports the first theoretical study on the collisional depolarization and polarization transfer for rovibrational levels of diatomic molecules in the infinite-order-sudden approximation. As an application, we compute depolarization and polarization transfer rates for rotational levels ,..., 99 within the vibrational quantum numbers , 1, 2, 3 of the SiO molecules in collisions with neutral hydrogen atoms for temperatures  K. Analytical expressions of the collisional depolarizing rates of rovibrational levels as a function of local temperature and rotational number J are obtained. We note that the collisional depolarization rates decrease when J increases and that the fractional effect of the collisions on the J-levels can be characterized by a simple ratio. Especially for high J values, we find that alignment and population transfer rates between J to inside the same vibrational number v are higher than the elastic depolarizing rates. This work might help to gain more understanding of the role of isotropic collisions in the formation of the very interesting polarized lines of C₂ and MgH observed in the solar photosphere.