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Published November 10, 2001 | Accepted Version + Published
Journal Article Open

H_2 and 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 Short-Wavelength Spectrometer observations of H_2 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_2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~ 10^(-4) M_☉ up to 8 x 10^(-3) and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish ^(12)CO 3-2/^(13)CO 3-2, and ^(12)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 ^(12)CO 3-2/^(13)CO 3-2 integrated fluxes indicate that ^(12)CO 3-2 is optically thick but that ^(13)CO 3-2 is optically thin or at most moderately thick. The ^(13)CO 3-2 lines have been used to estimate the cold gas mass. If a H_2/CO conversion factor of 1 x 10^(-4) 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_2 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 H_2 CO, or dust masses is found for stars with ages in the range of 10^6-10^7 yr, although a decrease is found for the older debris-disk star β Pictoris. The large amount of warm gas derived from H_2 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.

Additional Information

© 2001 American Astronomical Society. Received 2000 November 20; accepted 2001 July 17. Based in part on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries : France, Germany, Netherlands, and the United Kingdom) and with participation of ISAS and NASA. This work was supported by the Netherlands Organization for Scientific Research (NWO) grant 614.41.003 and a Spinoza grant, and by grants to G. A. B. from NASA (NAG 5-8822 and NAG 5-9434). A. N. is supported in part by ASI ARS-98-116 grant. Discussions with Eugene Chiang, Peter Goldreich, Michiel Hogerheijde, Frank Shu, and Doug Johnstone are appreciated. The authors thank the staff of the CSO and JCMT, in particular Fred Baas and Remo Tilanus, for their support and the Dutch ISO Data Analysis Center (DIDAC) at SRON-Groningen, especially Edwin Valentijn and Fred Lahuis, for their help during the data reduction of the ISO-SWS spectra.

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Published - 0004-637X_561_2_1074.pdf

Accepted Version - 0107006.pdf


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