Observations of molecular hydrogen in star-forming regions

Abstract

Highly excited molecular hydrogen is observed in dense molecular clouds in star-forming regions in our own and other galaxies. It is generated in shocks caused by powerful outflows from young or prestellar objects impacting the parent cloud, and also by fluorescence from recently formed massive stars irradiating their surroundings, and creating a photodissociation region.

It is usually supposed, on the basis of expected magnetic fields, that shocks in dense clouds ought to be (magnetically moderated) C shocks. Evidence is presented, based on H₂ measurements, that emission could be due to non-magnetic shocks, and that there is currently no entirely satisfactory theory for shocks in dense molecular clouds. This is a fruitful situation generating vigorous progress, and, once this problem is resolved, shock emission will be a powerful diagnostic of local chemistry, cloud structure and outflow properties.

Photodissociation regions have been observed in Orion, M17 and NGC 2023, and the existing theories compared. The physics of clumping, dynamical processes and chemistry all affect the predictions. The observations test the theory, and enable its extension to distant sources such as starburst galaxies.