The Excitation of SO in Cold Molecular Clouds: TMC‐1

Abstract

We have performed calculations on the excitation of SO using the accurate collisional rate coefficients at low temperatures for the system SO-He recently published by Lique et al. Compared with the results of Green, the new excitation rates show significant differences for transitions involving the low energy levels. We present a set of radiative transfer models for different cloud geometries and physical conditions, using the new rate coefficients for SO, and show that they provide a more correct diagnostic of the physical conditions of cold dark clouds. We have applied these results to observations of mapping results in TMC-1 done in the 23-12, 34-23, and 22-11 transitions (using the IRAM 30 m telescope) and the 01-10 transition (using the 100 m Effelsberg radio telescope). The intensity maxima for all these lines is found around the so-called ammonia peak. The structure of TMC-1 is well represented by a cylindric filament with several cores [TK = 8 K, n(H2) ~ 3 × 104 cm-3] surrounded by an envelope [TK = 10 K, n(H2) ~ (6-8) × 103 cm-3] showing a complex velocity structure. The abundance of SO in TMC-1 is nearly constant along the filament, X(SO) 10-8, except at the ammonia peak, where a higher SO abundance, X(SO) 2.5 × 10-8, is required.