Radiative trapping and population inversions of the SiO masers

Abstract

Radiative trapping in the v = 0 → 1 vibrational transitions decreases the rate of de-excitation of a v = 1 level as its rotational quantum number increases. This effect readily leads to population inversions of the v = 1 rotational transitions if population is transferred from v = 0 into v = 1 by collisions or by an indirect radiative route, as, for example, through absorptions from v = 0 into v = 2 and the subsequent spontaneous cascade from v = 2 to v = 1. Numerical calculations which model the SiO masers discovered toward stars give photon emission rates in the J = 1 → 0, J = 2 → 1, and J = 3 → 2 transitions which are comparable and substantial, 3 x 10^(43) s^(-1).

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