2007A&A...470..615P

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 (1996A&A...310..592S) to observations of the three lowest rotational transitions of ¹²CO, ¹³CO J=2-1, and J=3-2, as well as the [CI] ³P₁-³P₀ 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. 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 an external FUV field of 1-0.75χ₀, 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. The CO depletion only slightly reduces the ¹²CO and ¹³CO line intensity (by ≲10% and ≲20%, respectively). Predictions for the [CI] ²P_3/2-²P_1/2, [CI] ³P₂-³P₁, 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).