In situ probes of gaseous disks

Significant progress has been made over the few decades in developing in situ probes of the gaseous component of disks. As a result, a wide array of tools are now available for the study of gaseous disks over a wide range of disk radii. In the outer disk region (>10 AU), the available probes include millimeter molecular transitions, rovibrational transitions of H2 in the near-infrared (e.g., Bary et al. 2003), and atomic lines at optical wavelengths (e.g., Brandeker et al. 2004). In the giant planet region of the disk, we hope to use mid-infrared atomic and molecular transitions. At the smallest radii, in the terrestrial planet region of the disk, the available diagnostics now include near-infrared transitions of CO, OH, and water (e.g., Najita et al. 2000 for a review), as well as transitions of H2 at UV wavelengths (e.g., Herczeg et al. 2002). To illustrate the current state of affairs, I will highlight some of the successes and challenges associated with the use of these diagnostics in studying gaseous disks. Because of limited time, the discussion is representative rather than comprehensive, focusing on only one diagnostic from each region of the disk.

4.1. Kuiper Belt and beyond: Millimeter molecular transitions

Millimeter molecular transitions are the traditional probes of gas in the outer regions of circumstellar disks, and one of the most popular diagnostics are the pure rotational lines of CO (e.g., Sargent & Beckwith 1991; Dutrey et al. 1996). These transitions have been used with great success to show that the gaseous emission can be spatially resolved, typically on size scales >100 AU. Measurements of the spatially resolved velocity field of the emission further demonstrate that disks are rotating (e.g., Koerner 1995). Such measurements have been used with great success to measure both dynamical masses for the central stars, as well as system inclinations (e.g., Simon et al. 2000).

Due in part to the complex thermal-chemical structure of the disk, it has proven more difficult to estimate disk masses using these diagnostics. That is, the CO-observed emission is currently believed to arise from a warm layer at an intermediate height in the disk that is bounded from above by the photodissociation of CO, and from below by the condensation of CO in on cold grains in the disk midplane (e.g., Aikawa et al. 2002). Therefore, in order to derive a mass from the CO measurements, one needs to correct for the relative abundance of CO in the region where the emission arises, as well as to correct for the mass, primarily in the midplane, that is not directly probed by the CO emission. As a result of this complexity, disk masses have traditionally been estimated from the dust continuum emission instead (e.g., Beckwith et al. 1990).

Several surveys have been carried out using CO-rotational transitions for the purpose of studying the gas dissipation timescale in the outer region of the disk. One of the classics in this regard is the survey of Zuckerman et al. (1995), who inferred a dissipation timescale of <1 Myr based on CO transitions in a sample of young stars in the age range 1-10 Myr. At the time that the survey was carried out, the importance of depletion onto grains was just being recognized as an issue, which is therefore a caveat associated with the Zuckerman et al. study. More recent studies have continued to make progress in this area, e.g., the detection of strong CO emission from a weakly accreting young star (the T Tauri star V836 Tau—Duvert et al. 2000), and the demonstration that gas in the outer disk can survive for ^10 Myr around some Herbig AeBe stars (e.g., Dent et al. 2005). However, the limited sensitivity available to date has made it difficult to explore the commonality of CO emission from older and weakly accreting stars.

With the advent of the higher sensitivity and higher angular resolution available with ALMA, there is a bright future for resolving some of these difficulties. The use of diagnostics that specifically probe higher densities and smaller disk radii (e.g., HCO+ 4-3; Greaves 2004) may allow these studies to extend down to the smaller radii (<30 AU) more directly relevant to planet formation. Finally, detailed models of the thermal-chemical structure of disks that follow the vertical as well as the radial structure will be needed to derive robust gas masses from the observations.

4.2. Giant planet region: Mid-infrared transitions

At smaller radii, we hope to use the suite of transitions accessible in the mid-infrared in order to probe the properties of the gas in the giant planet region of the disk (~1-20 AU). These transitions include atomic fine-structure transitions, as well as transitions of molecules such as water, OH, and molecular hydrogen. The molecular hydrogen transitions are of particular interest, since molecular hydrogen dominates the gas mass and the pure rotational transitions in the mid-infrared are capable of probing the ^100 K temperatures expected for this region.

A recent study by Gorti & Hollenbach (2004) illustrates the ability of the transitions in this spectral region to probe the giant planet region in optically thin (debris) disks. Further work on the emission expected for optically thick disks is in progress. The results for optically thin disks illustrate some of the possible challenges associated with using these diagnostics to estimate gas masses. In these models, many of the diagnostics arise from a complex thermal-chemical structure that varies strongly with disk radius and vertical height (e.g., Hollenbach et al. 2005)—a situation similar to that found for the outer disk region. Since many of the diagnostics, including molecular hydrogen, arise from a restricted range of radii, significant corrections are needed to convert measured masses of warm H2 to total disk masses. Since the strengths of lines may depend on assumptions about the disk, such as the location of the inner disk radius (e.g., Hollenbach et al. 2005), high resolution spectra will likely be needed in order to break possible degeneracies and to reliably convert detected line strengths into disk masses.

Several surveys using the H2 pure rotational transitions have attempted to probe the gas dissipation timescale in the giant planet region of the disk. One of the most thought-provoking studies was one carried out with the Infrared Space Observatory (ISO), which reported the surprising detection of approximately Jupiter-masses of gas residing in ^20-Myr-old debris disk systems (Thi et al. 2001). This intriguing result is thus far unconfirmed from the ground (Richter et al. 2002; Sheret et al. 2003; Sako et al. 2005) or with Spitzer (e.g., Chen et al. 2004). The ground-based observations are typically carried out with higher angular resolution than the ISO observations, so a possible origin for the discrepancy between the results is that ISO was primarily sensitive to extended H2 emission not physically associated with the disk in the system.

Although the ground-based observations do not confirm the large line luminosities reported by ISO, H2 detections have been made (e.g., with the Texas Echelon Cross Echelle Spectrograph [TEXES] on the InfraRed Telescope Facility), albeit at lower flux levels than those reported by ISO. When detected, the emission is generally narrow, indicating that the emission arises from disk radii beyond 10 AU (M. Richter, personal communication). Thus, searches for the weaker emission that might originate from smaller radii, in the heart of the giant planet region of the disk, requires moving TEXES to a higher sensitivity platform such as the Gemini telescope or a future ground-based 30-m telescope.

4.3. Terrestrial planet region: CO fundamental emission At smaller disk radii, in the terrestrial planet region of the disk, the fundamental vi-brational transitions of CO are a possible probe of the gaseous disk. This diagnostic is appealing in that it is relatively common—it is detected in nearly all actively accreting, young low-mass stars (i.e., classical T Tauri stars; e.g., Najita et al. 2003). Fundamental emission is also detected among the higher mass Herbig AeBe stars (Brittain et al. 2003; Blake & Boogert 2004). The line profiles of the emission seen among the classical T Tauri stars (FWHM ^70 km s_1) show that the fundamental transitions probe the region of the disk from the inner disk edge at ^0.05 AU out to 1-2 AU, i.e., the terrestrial planet region of the disk. Since the emission spectrum includes lines covering a range of vibra-tional levels (e.g., v = 1-0, 2-1, 3-2), as well as lines of 13CO, the transitions together probe a wide range of temperatures and column densities (10~4 — 1gcm~2; Fig. 1).

The relative strengths of the emission lines show that they arise from surprisingly warm gas (^1000 K), much warmer than the dust temperatures expected for the terrestrial planet region (<400 K at 1 AU; e.g., D'Alessio et al. 1998). Recent models of the vertical thermal-chemical structure of the terrestrial planet region of disks surrounding T Tauri stars can account for these elevated gas temperatures (Glassgold et al. 2004). While the gaseous atmosphere is heated by stellar x-rays and by the mechanical heating that arises from accretion, the dust component is heated by longer wavelength stellar photons. This surface heating induces a vertical temperature inversion in both the gas and dust components. However, at the low densities characteristic of the upper atmosphere of the disk, the poor thermal coupling between the gas and dust components allows the gas to achieve a much higher temperature than the dust. As a result, spectral features that form in the temperature inversion region will appear in emission.

Looking at the chemistry that takes place within this thermal structure, we find that the transition from atomic carbon to CO takes place at a vertical hydrogen column den-

J. Najita: The evolution of gas in disks 111

i^vwuUUUUV^y

Figure 1. CO emission from classical T Tauri stars. Regions of strong telluric absorption have been excised from the plot. The centrally peaked line profiles show that the fundamental transitions probe a wide range of disk radii, from inner disk edge at —0.05 AU out to 1-2 AU, i.e., the terrestrial planet region of the disk. The emission spectrum can be rich (e.g., DF Tau), displaying lines from a range of vibrational levels (e.g., v = 1-0, 2-1, 3-2), as well as lines of 13CO.

sity of —1021 cm~2 and a temperature of —1000 K, similar to the observed temperatures and column densities characterizing the fundamental emission. Thus, these models plausibly explain circumstances under which the emission arises. Models of this kind will be needed in order to infer quantities such as total gas column densities from observed spectra (Fig. 2).

Since the CO fundamental emission probes the low column densities that are of interest for understanding the outcome of terrestrial planet formation, we (John Carr, Bob Mathieu, and I) have been attempting to use it to probe the residual gas content of weak and transitional T Tauri stars in the terrestrial planet region of the disk. Since these systems show little infrared excess in the terrestrial planet region of the disk, we can use our observations to explore whether significant gas might remain in the inner disk even after the inner disk has become optically thin in the continuum.

Thus far, we have several detections and many upper limits (Najita 2004; Fig. 3). The detections include the T Tauri star V836 Tau, a —3-Myr-old system (Siess et al. 1999), with an Ha equivalent width of 9 A (Herbig & Bell 1988), and an estimated accretion rate of —4 x 10~10 MQ yr^1 (Hartigan et al. 1995; Gullbring et al. 1998). CO emission is also detected from TW Hya (Fig. 3; see also Rettig et al. 2004, G. Blake personal communication), an —8-Myr-old T Tauri star with a mass accretion rate of 4 x 10-10 - 2 x 10~9 Mq yr^1 (Muzerolle et al. 2000; Alencar & Basri 2000). These results indicate the potential of using CO fundamental emission to probe the properties of residual gas disks.

There are several challenges to be faced in carrying out such a study. First, since stellar mass accretion rates are correlated with infrared excess (e.g., Kenyon & Hartmann 1995), detecting the presence of gas in systems with optically thin inner disks is nearly synonymous with detecting gas in systems with low accretion rates. As is known from our survey of classical T Tauri stars, the CO emission strength decreases with decreasing

| BP Tau

E- DF Tau

vJ vJ

Wavelength (um)

Wavelength (um)

1000

101B 1Q19 1020 1021 1022 1023 1024 1025 Vertical Column Density (cm-3)

Figure 2. Vertical temperature structure in a disk surrounding a classical T Tauri star at a radial distance of 1 AU (Glassgold et al. 2004). Both the gas and dust components experience vertical temperature inversions, with the gas achieving much higher temperatures than the dust. In the accompanying vertical chemical structure, a modest abundance of H2 (®(H2) = 10~3) is achieved at a vertical column density of ~1021 cm-2, which permits the conversion from atomic carbon to CO to occur at a similar column density. The gas temperature at this column density is comparable to the inferred excitation temperature of the CO fundamental emission that is detected from classical T Tauri stars. Full conversion to H2 is achieved at a much larger column density ~1022 cm-2. Similarly, a modest water abundance (®(H2O) = 3 x 10~6) is achieved at x 1021 cm-2, with an asymptotic abundance (®(H2O) = 3 x 10~4) achieved at much higher column densities (>1023 cm-2).

stellar mass accretion rate (Najita et al. 2003). This correlation might be due to the dissipation of the disk, i.e., that the total gas column density is decreasing as the accretion rate goes down. However, this seems unlikely, because the CO fundamental emission is sensitive to column densities of gas (^10~4-1 cm~2) much smaller than that of primordial gas disks (100-1000 gcm~2), and it seems unlikely that classical T Tauri stars have gas column densities much less than 1 gcm~2. It is more likely that the decrease in emission strength among the classical T Tauri stars is due to a heating effect, that the decrease in accretion rate results in less mechanical heating for the disk atmosphere and correspondingly weaker emission, despite a large gaseous reservoir.

Emission from the weak T Tauri stars is therefore expected to be even weaker than that found for the classical T Tauri stars, so high signal-to-noise spectra are required. Secondly, since these sources also have weak continuum excesses, any emission that we might detect will be superposed on the stellar photosphere. It may therefore be necessary to correct for the spectral structure in the stellar photosphere in order to detect any weak emission. An example of this is provided by the spectrum of TW Hya (Fig. 4),

Figure 2. Vertical temperature structure in a disk surrounding a classical T Tauri star at a radial distance of 1 AU (Glassgold et al. 2004). Both the gas and dust components experience vertical temperature inversions, with the gas achieving much higher temperatures than the dust. In the accompanying vertical chemical structure, a modest abundance of H2 (®(H2) = 10~3) is achieved at a vertical column density of ~1021 cm-2, which permits the conversion from atomic carbon to CO to occur at a similar column density. The gas temperature at this column density is comparable to the inferred excitation temperature of the CO fundamental emission that is detected from classical T Tauri stars. Full conversion to H2 is achieved at a much larger column density ~1022 cm-2. Similarly, a modest water abundance (®(H2O) = 3 x 10~6) is achieved at x 1021 cm-2, with an asymptotic abundance (®(H2O) = 3 x 10~4) achieved at much higher column densities (>1023 cm-2).

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