Photon Dominated Region Modeling of Barnard 68

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. We compare the spherically symmetric PDR model by Stoerzer, Stutzki & Sternberg (1996) to observations of the three lowest rotational transitions of 12CO, 13CO J = 2-1 and J = 3-2 as well as the [CI] 3P_1-3P_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 of the external FUV field. We find it difficult to simultaneously model the observed 12CO and 13CO emission. The 12CO and [CI] emission can be explained by a PDR model with a external FUV field of 1-0.75 chi_0, but this model fails to reproduce the observed 13CO by a factor of ~2. Adding PAHs to the chemical network increases the [CI] emission by 50% in our model but makes [CII] very faint. CO depletion only slightly reduces the 12CO and 13CO line intensity (by <10% and <20%, respectively). Predictions for the [CII] 2P_3/2-2P_1/2, [CI] 3P_2-3P_1 and 12CO J= 5-4 and 4-3 transitions are presented. This allows a test of our model with future observations (APEX, NANTEN2, HERSCHEL, SOFIA).