Kinetic temperatures in the Orion Bar

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 similar to 150 K. 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 (CO)-O-18, 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 (CO)-O-18 abundance ratio is 1.4 +/- 0.5 x 10(-2).