Molecular hydrogen imaging of star-forming regions

Observations of excited emission from molecular hydrogen have been used extensively for studies of star-forming regions for more than a decade. The characteristic response of the molecular gas to different excitation mechanisms, coupled with the in-depth theoretical modelling that has been carried out, make the near-infrared spectrum of H-2 a powerful diagnositic of the physical conditions in these regions. Early observations were able to establish whether molecular gas was excited by shocks or by radiation from young stars, using spectrometers with large beams. With improvements in theoretical understanding of the H-2 spectrum, observers were able to infer different density components within a molecular cloud from the NIR spectrum. Recent observations of the Orion Bar region made at the UK Infrared Telescope have combined Fabry-Perot imaging spectroscopy with observations at high angular resolution to examine the structure of the clouds in more detail. These observations reveal a complex filamentary structure on sub-arcsecond scales and provide direct evidence for high density clumps in the region.