1999A&A...349..253L

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 2.121µm for a region 5 arcsec in diameter. The line is resolved and the derived linewidth corresponds to a kinetic temperature of 500± 70K, 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, demonstrating the motion of the photodissociation front relative to the molecular cloud. Models of the photodissociation region experience only limited success in reproducing the known brightness of the H₂ emission in conjunction with the kinetic temperature determined here.