WARM NEUTRAL HALOS AROUND MOLECULAR CLOUDS .3. INTERPRETATION OF H-I AND CO-J = 1-0 DATA

A study is presented of the physical characteristics of the transition region separating the molecular and the atomic medium in interstellar clouds, based on H I 21 cm and CO j = 1-0 data. We find strong evidence for general heating of the surface layers of molecular clouds, and for the existence of large, heated neutral atomic halos around the molecular clouds. The heating of the molecular cloud surface is manifested through the general existence of limb brightening in the CO emission, with a characteristic depth of 1.7 pc. dependent on the Galactic latitude. He heated H I halos have a characteristic depth of 4.7 pc and a maximal extent of about 10 pc.