DETECTION OF LOW-J PURE-ROTATIONAL EMISSION FROM H-2 IN THE ORION BAR REGION - EVIDENCE FOR SMALL-SCALE CLUMPINESS

We present the results of a mapping of a 10" x 16" area in the vicinity of the Orion Bar ionization front, in the upsilon = 0 J = 3-1 and J = 4-2 emission lines of molecular hydrogen. This is the first detection of the J = 3-1 line in the interstellar medium and the first detection of any pure-rotational H-2 line in a photodissociation region. Since these lines are optically thin and thermalized, and the region is seen roughly edge-on, we are able to derive kinetic gas temperature and molecular column density distributions. When compared with models of photodissociation regions, our results support the scenario that the emission originates on the warm photodissociated surfaces of dense (greater-than-or-similar-to 10(6) cm-3) clumps of molecular gas embedded with a small volume filling factor in a low density (less-than-or-similar-to 5 x 10(4) cm-3) cooler atomic medium.