The detectability of mm-wave molecular rotational transitions

TL;DR

This paper presents a simplified formalism to assess the detectability of mm-wave molecular rotational lines, enabling easier interpretation of line brightness in weak excitation conditions without complex numerical modeling.

Contribution

It introduces a straightforward expression relating line brightness to gas and molecular densities, bypassing multi-level rate equations and optical depth calculations.

Findings

Valid for gas densities up to 10^4 cm^-3 and optical depths up to 100.

Provides upper bounds on line brightness for specific transitions.

Simplifies physical parameter derivation in weak excitation regimes.

Abstract

Elaborating on a formalism that was first expressed some 40 years ago, we consider the brightness of low-lying mm-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 \hcop, 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 {\it ansatz}, with the result that gas number densities as high as $10^4 \pccc$ 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 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.