MOLECULAR-HYDROGEN LINE EMISSION FROM THE REFLECTION NEBULA PARSAMYAN-18

The newly commissioned University of New South Wales Infrared Fabry-Pe rot (UNSWIRF) has been used to image molecular hydrogen emission at 2. 12 and 2.25 mu m in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1-0)/(2-1) S(1) ratio, suggestive of UV- pumped fluorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this fluorescent emission fr om extended are-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 fluxes and gas density are consistent w ith the predictions of photodissociation region (PDR) theory. Those re gions with the highest intrinsic 1-0 S(1) intensities also tend to sho w the highest (1-0)/(2-1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in the inc ident radiation field and/or the gas density, through the self-shieldi ng 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 re lative velocity, provides additional evidence for an outflow source as sociated with P 18.