OBSERVATION OF INFRARED AND RADIO LINES OF MOLECULES TOWARD GL-2591 AND COMPARISON TO PHYSICAL AND CHEMICAL-MODELS

We have observed rovibrational transitions of C2H2 and HCN near 13 mu m in absorption against GL 2591. We have marginally detected NH3 and set upper limits on rovibrational lines of CH4, CS, SO, and SiO. We have also observed rotational transitions at 0.6-3 mm of CS, HCN, H2CO, and HCO+. The rovibrational data were analyzed in comparison to the absorption line analysis of CO by Mitchell et al. (1989). Our data are consistent with the 2(C)H(2) and HCN absorption arising in the same warm (200 K) and hot (1010 K) components seen in CO, but we see little evidence for the cold (38 K) component seen in CO. The results can be explained by a model in which early-time gas-phase abundances are preserved on grain mantles and later released at high temperature. Analysis of the rotational lines indicates that these do not arise from the same gas as the rovibrational lines. Comparison of the two data sets shows that the rovibrational absorption of HCN must come from a region with a small angular extent (less than about 2-3 '', or about 2000-3000 AU at a distance of 1 kpc) and a much higher (factor of 400) abundance. The rovibrational lines from higher J states (J similar to 20) indicate that the hot HCN deviates from LTE. A good fit is obtained for a density of about 3 x 10(7) cm(-3). Analysis of the rotational lines, which arise in the extended cloud around the source, shows that no single-density model can explain all the data. Models with density and temperature gradients do much better; in particular models with n(r)proportional to r(-alpha), with alpha = 1.5, can reproduce the observed pattern of emission line strengths. Models with alpha = 1.0 or 2.0 are less satisfactory. These models predict densities of 3 x 10(7) cm(-3) at cm radii slightly smaller than, but similar to, the upper limits on the size derived above. The temperatures of the gas seen in the rovibrational lines are clearly higher than predicted from a similar extension of the temperature law, suggesting other sources of heating. Comparison of the radio and infrared data indicates that the line of sight to the infrared source has an unusually low column density.