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

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 2 3 -1 2 , 3 4 -2 3 , and 2 2 -1 1 transitions (using the IRAM 30 m telescope) and the 0 1 -1 0 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 [ T K = 8 K, n (H 2 ) ~ 3 × 10 4 cm -3 ] surrounded by an envelope [ T K = 10 K, n (H 2 ) ~ (6-8) × 10 3 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.