H2 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

By W. Thi, E. V. van Dishoeck, G. Blake, G. V. Zadelhoff, J. Horn, E. Becklin, V. Mannings, A. Sargent, M. E. V. D. Ancke, A. Natta and 10 more. 2001

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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) H2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~10-4 M☉ up to 8 × 10-3 M☉ and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish 12CO 3-2, 13CO 3-2, and 12CO 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 12CO 3-2/13CO 3-2 integrated fluxes indicate that 12CO 3-2 is optically thick but that 13CO 3-2 is optically thin or at most moderately thick. The 13CO 3-2 lines have been used to estimate the cold gas mass. If a H2/CO conversion factor of 1 × 104 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, H2 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.

Subfields: Astronomy and Astrophysics, Spectroscopy

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In Astronomy and Astrophysics

1. N. Evans II
2. D. Wilner
3. S. Andrews
4. E. Dishoeck
5. P. Caselli
6. E. Bergin
7. Simon Bruderer
8. P. Myers

In Spectroscopy

1. D. Wilner
2. S. Andrews
3. E. Dishoeck
4. P. Caselli
5. C. Dullemond
6. N. Evans II
7. E. Bergin
8. A. Hughes

Citations in the corpus

• A SPATIALLY RESOLVED VERTICAL TEMPERATURE GRADIENT IN THE HD 163296 DISK (2013, background): “The Herbig Ae star HD 163296 (spectral type A1) hosts a protoplanetary disk with bright continuum emission (Allen & Swings 1976; Mannings & Sargent 1997; Natta et al. 2004) and a rich molecular spectrum (Qi 2001; Thi et al. 2001, 2004; Qi et al. 2013).”
• [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation (2005, background): “Similar profiles have been observed in CO emission lines (e.g. Chandler et al. 1993; Thi et al. 2001) and have been attributed to the Keplerian rotation of the circumstellar gas disk.”
• The Spitzer c2d Survey of Weak-Line T Tauri Stars. II. New Constraints on the Timescale for Planet Building (2007, background): “Direct detection of forming planets is well beyond our current capabilities and observing molecular hydrogen, which largely dominates the mass of primordial disks, is particularly challenging (Thi et al. 2001, Richter et al. 2002).”
• Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars (2003, methodology): “Thus far, these lines have been detected in emission by the Infrared Space Observatory (ISO) in T Tauri stars and debris disk systems (Thi et al. 1999, 2001).”
• Imaging the Disk around TW Hydrae with the Submillimeter Array (2004, background): “Several molecular species (CO, HCN, CN, HCO+, DCO+) have been detected in the TW Hya disk at millimeter and submillimeter wavelengths using single dish telescopes (Kastner et al. 1997; van Zadelhoff et al. 2001; Thi et al. 2001; van Dishoeck et al. 2003).”
• Warm molecular layers in protoplanetary disks (2002, methodology): “…used to trace the gas and Keplerian velocity field in disks around classical T Tauri stars (e.g., Kawabe et al. 1993, Koerner et al. 1993, Dutrey et al. 1994, 1996, Koerner & Sargent 1995, Saito et al. 1995, Guilloteau & Dutrey 1998, Thi et al. 2001), detections of other molecules are still rare.”
• CO emission from discs around isolated HAeBe and Vega-excess stars (2005, unknown): “…et al. (2001); PDK03: Piétu, Dutrey, Kahane (2003); S01: Simon et al. (2001); SDB01: Sylvester, Dunkin & Barlow (2001); S96: Sylvester et al. (1996); SM00: Sylvester & Mannings (2000); T01: Thi et al. (2001); T04: Torres (2004); vdA98: van den Ancker et al. (1998); Z95: Zuckerman et al. (1995).”
• Rosseland and Planck mean opacities for protoplanetary discs (2003, background): “…observations of molecular lines provided basic information about chemical composition and dynamical properties of the gas in discs around pre-mainsequence stars and young stellar objects (e.g., Bujarrabalet al. 1997; Olofsson, Liseau, & Brandeker 2001; Thi et al. 2001; Piétu et al. 2003).”
• Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results (2010, background): “Sources have been selected to be clean from surrounding cloud emission in single-dish observations (e.g., Thi et al. 2001).”
• Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks (2006, background): “ISO observations detect H2 S(0) and S(1) emission with a line flux 6.6±2.0 × 10−14 ergs sec−1 cm−2 and place 3σ upper limits on the S(1) line flux, <0.8 × 10−14 ergs sec−1 cm−2, corresponding to a gas:dust ratio >100 (Thi et al. 2001).”
• The Far-Ultraviolet Spectra of TW Hydrae. II. Models of H2 Fluorescence in a Disk (2004, background): “Thi et al. (2001) detected cold H2 emission in the pure rotational S(0) and S(1) lines from ISO observations of a large sample of CTTSs, Herbig Ae/Be stars, and debris-disk stars.”
• ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks (2004, background): “Also, the gas temperature needs to be sufficiently high, >80 K. Deep searches for the H2 S(0) and S(1) lines at 28 and 17 µm have been performed with the SWS toward a dozen young stars (Thi et al. 2001a,b).”
• H2O and OH Gas in the Terrestrial Planet-forming Zones of Protoplanetary Disks (2008, background): “The NIRSPEC lines are narrow (FWHM. 35 km s−1), spatially unresolved, and cen- tered at the stellar vLSR (Herbig & Bell 1988; Thi et al. 2001) to within our uncertainty (a few km s−1) — all consistent with disk emission.”
• Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars (2004, methodology): “The sources were selected to have strong CO J = 3→ 2 fluxes and the highest number of molecular lines detected in previous observations (Qi 2001; Thi et al. 2001, van Zadelhoff et al. 2001).”
• The Far-Ultraviolet Spectrum of TW Hydrae. I. Observations of H2 Fluorescence (2002, methodology): “On the basis of submillimeter CO emission and estimates for the disk mass, Thi et al. (2001) and van Zadelhoff et al. (2001) determined that CO is underabundant by a factor of 500 around TWHya, with respect to a typical interstellar medium (ISM) H2/CO ratio of 104.”