THE DETECTABILITY OF MILLIMETER-WAVE MOLECULAR ROTATIONAL TRANSITIONS

Abstract

Elaborating on a formalism that was first expressed some 40 years ago, we consider the brightness of low-lying millimeter-wave rotational lines of strongly polar molecules at the threshold of detectability. We derive a simple expression relating the brightness to the line-of-sight integral of the product of the total gas and molecular number densities and a suitably defined temperature-dependent excitation rate into the upper level of the transition. Detectability of a line is contingent only on the ability of a molecule to channel enough of the ambient thermal energy into the line, and the excitation can be computed in bulk by summing over rates without solving the multi-level rate equations, or computing optical depths and excitation temperatures. Results for , HNC, and CS are compared with escape-probability solutions of the rate equations using closed-form expressions for the expected range of validity of our ansatz, with the result that gas number densities as high as or optical depths as high as 100 can be accommodated in some cases. For densities below a well-defined upper bound, the range of validity of the discussion can be cast as an upper bound on the line brightness which is 0.3 K for the J = 1–0 lines and 0.8–1.7 K for the J = 2–1 lines of these species. The discussion casts new light on the interpretation of line brightnesses under conditions of weak excitation, simplifies derivation of physical parameters, and eliminates the need to construct grids of numerical solutions of the rate equations.