Molecular Hydrogen Line Emission from the Re ectionNebula

The newly-commissioned University of New South Wales Infrared Fabry-Perot (UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25 m in the reeection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1 ? 0)=(2 ? 1) S(1) ratio suggestive of UV-pumped uorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this uores-cent emission from extended arc-like features, as well as a more concentrated thermal component in regions closer to the central exciting star. We show that the emission morphology, line uxes, and gas density are consistent with the predictions of pho-todissociation region (PDR) theory. Those regions with the highest intrinsic 1?0 S(1) intensities also tend to show the highest (1 ? 0)=(2 ? 1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic uctuations in the incident radiation eld and/or the gas density, through the self-shielding action of H 2. An isolated knot of emission discovered just outside P 18, and having both an unusually high (1 ? 0)=(2 ? 1) S(1) ratio and relative velocity provides additional evidence for an outtow source associated with P 18.