We present a direct comparison of a chemical/physical model to multitransitional observations of C ^ { 18 } O and ^ { 13 } CO towards the Barnard 68 pre-stellar core .
These observations provide a sensitive test for models of low UV field photodissociation regions and offer the best constraint on the gas temperature of a pre-stellar core .
We find that the gas temperature of this object is surprisingly low ( \sim 7 - 8 K ) , and significantly below the dust temperature , in the outer layers ( A _ { V } < 5 mag ) that are traced by C ^ { 18 } O and ^ { 13 } CO emission .
As shown previously , the inner layers ( A _ { V } > 5 mag ) exhibit significant freeze-out 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 \sim 1 K. The gas temperature in layers not traced by C ^ { 18 } O and ^ { 13 } CO emission can be probed by NH _ { 3 } emission , with a previously estimated temperature of \sim 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 .