Heating the Gas in Photodissociation Fronts
Abstract
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 H2 molecule. The H2 rotational level populations provide a thermal probe, showing that gas temperatures Tgas approx 500-1000 K prevail in a portion of the PDR where significant quantities of H2 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.
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