The thermal structure of gas in prestellar cores: A case study of Barnard 68

We present a direct comparison of a chemical/physical model to multitransitional observations of (CO)-O-18 and (CO)-C-13 toward the Barnard 68 prestellar core. These observations provide a sensitive test for models of low UV field photo-dissociation regions and offer the best constraint on the gas temperature of a prestellar core. We find that the gas temperature of this object is surprisingly low (similar to 7-8 K), and significantly below the dust temperature, in the outer layers (A(V) < 5 mag) that are traced by (CO)-O-18 and (CO)-C-13 emission. As shown previously, the inner layers (A(V) > 5 mag) exhibit significant freezeout of CO onto grain surfaces. Because the dust and gas are not fully coupled, depletion of key coolants in the densest layers raises the core (gas) temperature, but only by similar to 1 K. The gas temperature in layers not traced by (CO)-O-18 and (CO)-C-13 emission can be probed by NH3 emission, with a previously estimated temperature of similar to 10-11 K. To reach these temperatures in the inner core requires an order of magnitude reduction in the gas to dust coupling rate. This potentially argues for a lack of small grains in the densest gas, presumably due to grain coagulation.