Detection of low-J pure-rotational emission from H2 in the Orion Bar region : evidence for small-scale clumpiness

The paper presents the results of a mapping of a 10 x 16 arcsec area in the vicinity of the Orion Bar ionization front, in the v = 0 J = 3{minus}1 and J = 4{minus}2 emission lines of molecular hydrogen. This is the first detection of the J = 3{minus}1 line in the interstellar medium and the first detection of any pure-rotational H2 line in a photodissociation region. Since these lines are optically thin and thermalized, and the region is seen roughly edge-on, kinetic gas temperature and molecular column density distributions can be derived. When compared with models of photodissociation regions, the results support the scenario that the emission originates on the warm photodissociated surfaces of dense clumps of molecular gas embedded with a small volume filling factor in a low density cooler atomic medium. 23 refs.