PHYSICAL AND CHEMICAL-STRUCTURE OF THE IC-63 NEBULA .1. MILLIMETER AND FAR-INFRARED OBSERVATIONS

We present results of a (sub)millimeter and far-infrared study of the reflection/emission nebula IC 63, located close to the B0.5p star gamm a Cas. The source has been mapped in the (CO)-C-12 2 --> 1 and 3 --> 2 , (CO)-C-13 2 --> 1, and CS 2 --> 1 lines and shows a small molecular cloud less than 1' x 2' in extent, which coincides with the brightest optical nebulosity and IRAS 100 mum emission. IC 63 is therefore an ex cellent example of a nearby (d almost-equal-to 230 pc), edge-on photon -dominated region (PDR). Various other molecules have been observed at the peak position through their rotational transitions, in order to p robe the physical parameters and to derive abundances. The measured CO , HCO+, HCN, CS and H2CO line ratios suggest that the cloud is warm, T almost-equal-to 50 K, and dense, n(H-2) almost-equal-to 5 x 10(4) cm- 3. Excitation of molecules by electrons may play a significant role in this PDR. On the basis of these physical conditions, column densities have been determined from the observed line strengths. Several differ ent methods are discussed to constrain the H-2 column density, includi ng the use of measured submillimeter continuum fluxes. The resulting a bundances of species such as CN and CS are similar to those found in c old, dark clouds like TMC-1 and L134N. However, the abundances of othe r simple molecules such as HNC, HCO+, N2H+ and possibly C2H are lower by factors of at least three, probably because of the enhanced photodi ssociation rates at a distance of 1.3 pc from a B star. Surprisingly, only the abundance of the H2S molecule appears enhanced. More complex, volatile molecules such as CH3OH, CH3CN and HNCO, and the sulfur-oxid es SO and SO2 have not been found in this cloud. Limited observations of molecules in the reflection nebula NGC 2023 are presented as well, and the resulting molecular abundances are compared with those found f or IC 63.