Physical and chemical structure of the IC 63 nebula. II. Chemical models.

Abstract

Chemical models are presented for the photon dominated region (PDR) IC 63, a small isolated molecular cloud located close to the B0.5 IVpe star γ Cas for which we have presented observations of various molecular species in an earlier paper, and for which observations of ionized and neutral atomic carbon, as well as additional CO observations, are available. The models treat the photoexcitation and photodissociation processes in detail, and use the actual γ Cas radiation field as measured by the S2-68 and IUE satellites. The heating and cooling balance of the cloud is taken explicitly into account. The computed H_2_ ultraviolet fluorescent spectrum agrees well with observations, thus providing support for the inferred physical structure. Effects of the radiation field, elemental depletions, density and cosmic ray ionization rate on the chemistry are discussed. Several distinct chemical zones are found with depth into the cloud. At the edge, most species are in ionized atomic form, and reactions with C^+^ drive the chemistry. Deeper into the cloud around A_V_=~2 mag, carbon is transformed into neutral atomic carbon and CO. At this same depth, the abundances of radicals such as CH, CH_2_, CH_3_, CN and C_2_H peak. They rapidly decline at larger depths due to reactions with atomic oxygen. Only species such as CH_4_, H_2_CO, and HCN, which do not react with O, have large abundances deep into the cloud. As a result, the CN/HCN abundance ratio varies strongly with depth. Most observed column densities can be reproduced to within a factor of a few, and in many cases even better than a factor of two. Only the chemistry of sulfur-bearing molecules does not fit the observations: if the sulfur depletion is fixed to reproduce the observed CS column density, the model H_2_S column density is too low and that of SO too high by an order of magnitude. The resulting temperature structure in the one-dimensional plane-parallel models agrees reasonably well with that inferred from observations. Improved comparison is obtained in two-dimensional models which take the actual geometry of the region into account. The results of such geometrical models are presented, and are compared with those obtained in the one-dimensional case. In general, the agreement between one-dimensional and two-dimensional models is good enough to justify the one-dimensional approximation in a simple cloud like IC 63, whereas for more complex clouds the actual geometry might become very important.