2001ApJ...561.1074T

We present ISO Short-Wavelength Spectrometer observations of H₂ pure-rotational line emission from the disks around low- and intermediate-mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. The pre-main-sequence sources have been selected to be isolated from molecular clouds and to have circumstellar disks revealed by millimeter interferometry. We detect ``warm'' (T~100-200 K) H<sub>2</sub> gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ∼10<sup>–4</sup> M<sub>☉</sub> up to 8x10<sup>–3</sup> M<sub>☉</sub> and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish <sup>12</sup>CO 3-2, <sup>13</sup>CO 3-2, and <sup>12</sup>CO 6-5 observations have 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 disk in Keplerian rotation, consistent with interferometer data on the lower J lines. The ratios of the <sup>12</sup>CO 3-2/<sup>13</sup>CO 3-2 integrated fluxes indicate that <sup>12</sup>CO 3-2 is optically thick but that <sup>13</sup>CO 3-2 is optically thin or at most moderately thick. The <sup>13</sup>CO 3-2 lines have been used to estimate the cold gas mass. If a H<sub>2</sub>/CO conversion factor of 1x10<sup>4</sup> is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100, in accordance with previous studies. These findings confirm that CO is not a good tracer of the total gas content in disks since it can be photodissociated in the outer layers and frozen onto grains in the cold dense part of disks, but that it is a robust tracer of the disk velocity field. In contrast, H<sub>2</sub> can shield itself from photodissociation even in low-mass ``optically thin'' debris disks and can therefore survive longer. The warm gas is typically 1%-10% of the total mass deduced from millimeter continuum emission, but it can increase up to 100% or more for the debris-disk objects. 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 with ages in the range of 10⁶-10⁷ yr, although a decrease is found for the older debris-disk star β Pictoris. The large amount of warm gas derived from H₂ raises the question of the heating mechanism(s). Radiation from the central star as well as the general interstellar radiation field heat an extended surface layer of the disk, but existing models fail to explain the amount of warm gas quantitatively. The existence of a gap in the disk can increase the area of material influenced by radiation. Prospects for future observations with ground- and space-borne observations are discussed.