1999ApJ...515..649K

Infrared Space Observatory (ISO) [C II] 158 µm, [O I] 63 µm, and H₂ 9 and 17 µm observations are presented of the reflection nebula Ced 201, which is a photon-dominated region (PDR) illuminated by a B9.5 star with a color temperature of 10,000 K (a cool PDR). In combination with ground-based [C I] 609 µm, CO, ¹³CO, CS, and HCO⁺ data, the carbon budget and physical structure of the reflection nebula are constrained. The obtained data set is the first one to contain all important cooling lines of a cool PDR and allows a comparison to be made with classical PDRs. To this effect one- and three-dimensional PDR models are presented that incorporate the physical characteristics of the source and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to the photoelectric heating rate is estimated from these models and is used to constrain the total abundance of polycyclic aromatic hydrocarbons and small grains. Observations of the pure rotational H₂ lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm ∼330 K molecular gas. This gas cannot be accommodated by the presented models.