SIMBAD references

We report the detection of the (J, K)=(1, 1), (2, 2) and (4, 4) inversion lines of ammonia towards the Photo Dissociation Region (PDR) of the Orion Bar. The NH₃ emission is found in the same location as emission from HCN. Temperatures determined from line intensity ratios indicate a warm gas with T_k of ∼150K. This value is inconsistent with temperatures predicted from models of the gas phase chemistry of dense PDRs. We propose that the emission arises from ammonia released into the gas phase from the evaporation of dense clumps. From measurements of the J=2-1 line of C¹⁸O, we obtain beam-averaged column densities for those positions where the (1, 1), (2, 2) and (4, 4) inversion lines were measured. The ammonia to C¹⁸O abundance ratio is 1.4±0.5x10^–2.