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

Abstract

We present the results of a mapping of a 10″ × 16″ area in the vicinity of the Orion Bar ionization front, in the υ = 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 H2 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 (≳106 cm-3) clumps of molecular gas embedded with a small volume filling factor in a low density (≲5 × 104 cm-3) cooler atomic medium.