MOLECULAR LINE EMISSION FROM CIRCUMSTELLAR DISKS

Calculations of the molecular line emission from circumstellar disks around young stars show the transitions of CO and its CoMMon isotopes (CO)-C-13 and (CO)-O-18 to have typical line-center optical depths between 100 and 1000. The line emission reflects the temperature distribution within the disks and not the mass distribution as previously suggested. The (CO)-C-13 line fluxes from the HL Tau and two similar stars show that the outer disk temperatures are, indeed, higher than expected for normal accretion disks, as inferred from far-infrared continuum data. The line radiation indicates that the disks are at least 400 AU in extent. The very high optical depths in the CO lines imply that as little as 6 x 10(-6) M. of gas may be detected around very young stars with existing instrumentation.