Heating the Gas in Photodissociation Fronts

ISO has provided us with a new perspective on gas temperatures in photodissociation regions through measurements of line emission from rotationally-excited levels of the H_2 molecule. The H_2 rotational level populations provide a thermal probe, showing that gas temperatures T_gas approx 500-1000 K prevail in a portion of the PDR where significant quantities of H_2 are present. Such high gas temperatures were unexpected. Theoretical models for the S140 PDR are presented. Possible mechanisms for heating the gas to such high temperatures are discussed.