Submillimeter and far-infrared spectroscopy of M17 and S106 - UV-heated, quiescent molecular gas?

The authors report measurements of 372 μm J = 7→6 and 186 μm J = 14→13 CO line emission toward the interface between the molecular cloud and the H II region in M17, and toward the center of the bipolar nebula S106. The ratio of 14→13/7→6 lines indicates gas temperatures between 200 and 500 K, and hydrogen densities ≥10⁴cm⁻³. The 7→6 lines are remarkably narrow, with widths of ≡5 - 10 km s⁻¹, identical to those of the cool quiescent molecular cloud cores. The warm quiescent molecular gas is in the interface between the exciting OB stars and the surrounding molecular cloud. Possible heating mechanisms of the warm CO gas are slow shocks and heating by photoelectrons in UV-illuminated, photodissociation regions.