H2and CO Emission from Disks around T Tauri and Herbig Ae Pre–Main‐Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas

We present ISO-SWS observations of H2 pure-rotational line emission from thedisks around low and intermediate mass pre-main-sequence stars as well as fromyoung stars thought to be surrounded by debris disks. We detect `warm' (T ~100-200 K) H2 gas around many sources, including tentatively the debris-diskobjects. The mass of this warm gas ranges from ~1E-4 Solar mass up to 8E-3Solar mass, and can constitute a non-negligible fraction of the total diskmass. Complementary single-dish 12CO 3-2, 13CO 3-2 and 12CO 6-5 observationshave been obtained as well. These transitions probe cooler gas at T ~ 20-80 K.Most objects show a double-peaked CO emission profile characteristic of a diskin Keplerian rotation, consistent with interferometer data on the lower-Jlines. The ratios of the 12CO 3-2/ 13CO 3-2 integrated fluxes indicate that12CO 3-2 is optically thick but that 13CO 3-2 is optically thin or at mostmoderately thick. The 13CO 3-2 lines have been used to estimate the cold gasmass. If a H2/CO conversion factor of 1E4 is adopted, the derived cold gasmasses are factors of 10-200 lower than those deduced from 1.3 millimeter dustemission assuming a gas/dust ratio of 100,in accordance with previous studies.The warm gas is typically 1-10 % of the total mass deduced from millimetercontinuum emission, but can increase up to 100% or more for the debris-diskobjects. Thus, residual molecular gas may persist into the debris-disk phase.No significant evolution in the H2, CO or dust masses is found for stars withages in the range of 1E6-1E7 years, although a decrease is found for the olderdebris-disk star beta Pictoris. Existing models fail to explain the amount ofwarm gas quantitatively.Comment: 49 pages, 14 figures, accepted for publication in ApJ main journa