Abstract
The detection of extragalactic OCS, the heaviest molecule identified outside the Galaxy, is reported in three rotational transitions toward the bulge of NGC 253. Toward M 82, OCS has been tentatively detected. Abundances are consistent with models for warm, dense clouds which have not yet reached chemical equilibrium or with the frequent erosion of grain mantles by C-shocks. This can also explain the large abundances of complex molecules in NGC 253.
Densities and temperatures of the OCS emitting gas are compared with results from other molecular tracers. From model computations, OCS in the center of NGC 253 is emitted by a moderately dense molecular component with n(H-2) similar to 10(3.4)cm(-3). This component makes up the bulk of the molecular gas mass. Other molecules also reveal the presence of a much denser component with 10(5-5.5)cm(-3). A molecular gas mass of similar to 2 10(8) Mg is estimated for a 180 pc sized molecular cloud toward the center of NGC 253; the mean H-2 density is similar to 2000 cm(-3); and the mean H-2 column density, 7 10(23) cm(-2) corresponds to a visual extinction of 700(m). The relative OCS abundance is X(OCS) similar to 10(-8.9).
From detections of the J = 2 - 1 and 3 - 2 transitions of (CS)-S-34, the density of the dense molecular gas toward the nuclear region of IC342 is n(H-2) similar to 10(4.6)cm(-3). Also presented are a possible detection of the HC3N J = 16 - 15 line toward IC 342 and upper limits for the emission of the DCN J = 2 - 1 line toward NGC 253 and IC 342. For NGC 253, [DCN]/[HCN] less than or equal to 410(-3).