We present an analysis of Spitzer-IRS observations of H2O, OH, HCN, C2H2, and CO2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of
most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions. We note that the line detection efficiency is anti-correlated with the 13/30 um SED spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and H-alpha equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H2O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L_star. However, significant sample variation, especially in molecular line ratios, remains. LTE models of the H2O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H2O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near 10^{18} cm-2 and 450 K for all sources, suggesting a high abundance of H2O in many planet-forming regions. Other molecules have a range of excitation temperatures from ~500-1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ~10^{-3} for all molecules, with the exception of CO, for which n(CO)/n(H2O)~1. However, LTE fitting caveats and differences in the way thermo-chemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.
Mid-infrared spectrophotometric observations have revealed a small sub-class of circumstellar disks with spectral energy distributions (SEDs) suggestive of large inner gaps with low dust content. However, such data provide only an indirect and model-
dependent method of finding central holes. Imaging of protoplanetry disks provides an independent check of SED modeling. We present here the direct characterization of three 33-47 AU radii inner gaps, in the disks around LkHa 330, SR 21N and HD 135344B, via 340 GHz (880 micron) dust continuum aperture synthesis observations obtained with the Submillimeter Array (SMA). The large gaps are fully resolved at ~0farcs3 by the SMA observations and mostly empty of dust, with less than 1 - 7.5 x 10^-6 Msolar of fine grained solids inside the holes. Gas (as traced by atomic accretion markers and CO 4.7 micron rovibrational emission) is still present in the inner regions of all three disks. For each, the inner hole exhibits a relatively steep rise in dust emission to the outer disk, a feature more likely to originate from the gravitational influence of a companion body than from a process expected to show a more shallow gradient like grain growth. Importantly, the good agreement of the spatially resolved data and spectrophotometry-based models lends confidence to current interpretations of SEDs, wherein the significant dust emission deficits arise from disks with inner gaps or holes. Further SED-based searches can therefore be expected to yield numerous additional candidates that can be examined at high spatial resolution.
The study of warm molecular gas in the inner regions of protoplanetary disks is of key importance for the study of planet formation and especially for the transport of H2O and organic molecules to the surfaces of rocky planets/satellites. Recent Spit
zer observations have shown that the mid-infrared spectra of protoplanetary disks are covered in emission lines due to water and other molecules. Here, we present a non-LTE 2D radiative transfer model of water lines in the 10-36 mum range that can be used to constrain the abundance structure of water vapor, given an observed spectrum, and show that an assumption of local thermodynamic equilibrium (LTE) does not accurately estimate the physical conditions of the water vapor emission zones. By applying the model to published Spitzer spectra we find that: 1) most water lines are subthermally excited, 2) the gas-to-dust ratio must be one to two orders of magnitude higher than the canonical interstellar medium ratio of 100-200, and 3) the gas temperature must be higher than the dust temperature, and 4) the water vapor abundance in the disk surface must be truncated beyond ~ 1 AU. A low efficiency of water formation below ~ 300 K may naturally result in a lower water abundance beyond a certain radius. However, we find that chemistry, may not be sufficient to produce an abundance drop of many orders of magnitude and speculate that the depletion may also be caused by vertical turbulent diffusion of water vapor from the superheated surface to regions below the snow line, where the water can freeze out and be transported to the midplane as part of the general dust settling. Such a vertical cold finger effect is likely to be efficient due to the lack of a replenishment mechanism of large, water-ice coated dust grains to the disk surface.
Mid-infrared spectrophotometric observations have revealed a small sub-class of circumstellar disks with spectral energy distributions (SEDs) suggestive of large inner gaps with low dust content. However, such data provide only an indirect and model
dependent method of finding central holes. We present here the direct characterization of a 40 AU radius inner gap in the disk around LkHa 330 through 340 GHz (880 micron) dust continuum imaging with the Submillimeter Array (SMA). This large gap is fully resolved by the SMA observations and mostly empty of dust with less than 1.3 x 10^-6 M_solar of solid particles inside of 40 AU. Gas (as traced by accretion markers and CO M-band emission) is still present in the inner disk and the outer edge of the gap rises steeply -- features in better agreement with the underlying cause being gravitational perturbation than a more gradual process such as grain growth. Importantly, the good agreement of the spatially resolved data and spectrophometry-based model lends confidence to current interpretations of SEDs with significant dust emission deficits as arising from disks with inner gaps or holes. Further SED-based searches can therefore be expected to yield numerous additional candidates that can be examined at high spatial resolution.
We present detections of numerous 10-20 micron H2O emission lines from two protoplanetary disks around the T Tauri stars AS 205A and DR Tau, obtained using the InfraRed Spectrograph on the Spitzer Space Telescope. Follow-up 3-5 micron Keck-NIRSPEC da
ta confirm the presence of abundant water and spectrally resolve the lines. We also detect the P4.5 (2.934 micron) and P9.5 (3.179 micron) doublets of OH and 12CO/13CO v=1-0 emission in both sources. Line shapes and LTE models suggest that the emission from all three molecules originates between ~0.5 and 5 AU, and so will provide a new window for understanding the chemical environment during terrestrial planet formation. LTE models also imply significant columns of H2O and OH in the inner disk atmospheres, suggesting physical transport of volatile ices either vertically or radially; while the significant radial extent of the emission stresses the importance of a more complete understanding of non-thermal excitation processes.
We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks Legacy Program Infrared Spectrogr
aph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation.
