Far-infrared and submillimeter observations and physical models of the reflection nebula Cederblad 201

Infrared Space Observatory (ISO) CC nl 158 mu m, [O I] 63 mu m, and H-2 9 a nd 17 mu m observations are presented of the reflection nebula Ced 201, whi ch is a photon-dominated region (PDR) illuminated by a B9.5 star with a col or temperature of 10,000 K (a cool PDR). In combination with ground-based [ C I] 609 mu m, CO, (CO)-C-13, CS, and HCO+ data, the carbon budget and phys ical 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 a nd allows a comparison to be made with classical PDRs, To this effect one- and three-dimensional PDR models are presented that incorporate the physica l characteristics of the source and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to t he photoelectric heating rate is estimated from these models and is used to constrain the total abundance of polycyclic aromatic hydrocarbons and smal l grains. Observations of the pure rotational H-2 lines with ISO, in partic ular the S(3) line, indicate the presence of a small amount of very warm si milar to 330 K molecular gas. This gas cannot be accommodated by the presen ted models.