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High resolution Fourier transform spectroscopy of H 2 IR emission in NGC 7023

A Fourier transform spectrum at a resolution of 125,000 has been recorded of molecular hydrogen emission in the reflection nebula NGC 7023, in the S(1) v=1-0 line at 2.121 m for a region 5 arcsec in diameter. The line is resolved and the derived linewidth corresponds to a kinetic temperature of 500 70 K, if all motions are thermal. Comparison with data for HI, CI, CII and CO shows that there is a velocity gradient in the molecular material in the photodissociation region, demon- strating the motion of the photodissociation front relative to the molecular cloud. Models of the photodissociation region expe- rience only limited success in reproducing the known brightness of the H2 emission in conjunction with the kinetic temperature determined here.