Heat sources for bright-rimmed molecular clouds: CO observations of NGC 782.

A (C-12)O map of the bright rim in NGC 7822 is presented, and several observations of this bright-rimmed molecular cloud (BRMC) in (C-13)O are used to determine column densities. The results reveal the presence of a plateau of enhanced (C-12)O temperature and column density inside the bright rim. Several possible heat sources that may be important for this cloud and other BRMCs are investigated, including cosmic rays, shock-wave energy deposition, viscous gas heating by Alfven waves or turbulence, and heating by radiation from the Cep IV OB association through grain-related processes. It is found that cosmic rays provide an excellent heat source for Bok globules but not for BRMCs, that shock heating would be adequate only if the internal pressure in NGC 7822 far exceeds the thermal pressure, that grain heating by radiation from nearby OB stars would be insignificant, and that relaxation of internal magnetic fields via ion-neutral collisions is the most reasonable source of internal heating. Heating from unknown embedded stars is shown to be an obvious alternative to the energy sources considered.