Photon-dominated region modeling of the CO and [CI] line emission in Barnard 68

Aims. We use the Barnard 68 dark globule as a test case for a spherically symmetric PDR model exposed to low- UV radiation fields. With a roughly spherical morphology and an accurately determined density profile, Barnard 68 is ideal for this purpose. The processes governing the energy balance in the cloud surface are studied in detail. Methods. We compare the spherically symmetric PDR model by Storzer, Stutzki & Sternberg ( 1996) to observations of the three lowest rotational transitions of (CO)-C-12, (CO)-C-13 J = 2 -> 1, and J = 3 -> 2, as well as the [C I] P-3(1)-> P-3(0) fine structure transition. We study the role of polycyclic aromatic hydrocarbons (PAHs) in the chemical network of the PDR model and consider the impact of depletion, as well as of a variation in the external FUV field. Results. We find it di. cult to simultaneously model the observed (CO)-C-12 and (CO)-C-13 emission. The (CO)-C-12 and [C I] emission can be explained by a PDR model with an external FUV field of 1-0.75.0, but this model fails to reproduce the observed (CO)-C-13 by a factor of similar to 2. Adding PAHs to the chemical network increases the [C I] emission by 50% in our model but makes [C II] very faint. The CO depletion only slightly reduces the (CO)-C-12 and (CO)-C-13 line intensity (by less than or similar to 10% and less than or similar to 20%, respectively). Predictions for the [CI] P-2(3)/(2)-> P-2(1/2) 2, [CI] (P2 -> P1)-P-3-P-3, and (CO)-C-12 J = 5 -> 4 and 4 -> 3 transitions are presented. This allows a test of our model with future observations (APEX, NANTEN2, HERSCHEL, SOFIA).