Circumstellar disks are thought to experience a rapid "transition" phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. We present new and archival high angular resolution (0. ′′ 3 ≈ 40-75 AU) Submillimeter Array (SMA) observations of the 880 µm (340 GHz) dust continuum emission from 12 such transition disks in nearby star-forming regions. In each case, we directly resolve a dust-depleted disk cavity around the central star. Using two-dimensional Monte Carlo radiative transfer calculations, we interpret these dust disk structures in a homogeneous, parametric model framework by reproducing their SMA continuum visibilities and spectral energy distributions. The cavities in these disks are large (R cav = 15-73 AU) and substantially depleted of small (∼µm-sized) dust grains, although their mass contents are still uncertain. The structures of the remnant material at larger radii are comparable to normal disks. We demonstrate that these large cavities are relatively common among the millimeter-bright disk population, comprising at least 1 in 5 (20%) of the disks in the bright half (and ≥26% of the upper quartile) of the millimeter luminosity (disk mass) distribution. Utilizing these results, we assess some of the physical mechanisms proposed to account for transition disk structures. As has been shown before, photoevaporation models do not produce the large cavity sizes, accretion rates, and disk masses representative of this sample. It would be difficult to achieve a sufficient decrease of the dust optical depths in these cavities by particle growth alone: substantial growth (to meter sizes or beyond) must occur in large (tens of AU) regions of low turbulence without also producing an abundance of small particles. Given those challenges, we suggest instead that the observations are most commensurate with dynamical clearing due to tidal interactions with low-mass companions -young brown dwarfs or giant planets on long-period orbits. Subject headings: circumstellar matter -protoplanetary disks -planet-disk interactions -planets and satellites: formation -submillimeter: planetary systems

We present the results of a high angular resolution (0. 3 ≈ 40 AU) Submillimeter Array survey of the 345 GHz (870 μm) thermal continuum emission from nine of the brightest, and therefore most massive, circumstellar disks in the ∼1 Myr-old Ophiuchus star-forming region. Using two-dimensional radiative transfer calculations, we simultaneously fit the observed continuum visibilities and broadband spectral energy distribution for each disk with a parametric structure model. Compared to previous millimeter studies, this survey includes significant upgrades in modeling, data quality, and angular resolution that provide improved constraints on key structure parameters, particularly those that characterize the spatial distribution of mass in the disks. In the context of a surface density profile motivated by similarity solutions for viscous accretion disks,, the bestfit models for the sample disks have characteristic radii R c ≈ 20-200 AU, high disk masses M d ≈ 0.005-0.14 M (a sample selection bias), and a narrow range of radial Σ gradients (γ ≈ 0.4-1.0) around a median γ = 0.9. These density structures are used in conjunction with accretion rate estimates from the literature to help characterize the viscous evolution of the disk material. Using the standard prescription for disk viscosities, those combined constraints indicate that α ≈ 0.0005-0.08. Three of the sample disks show large (R ≈ 20-40 AU) central cavities in their continuum emission morphologies, marking extensive zones where dust has been physically removed and/or has significantly diminished opacities. Based on the current requirements of planet formation models, these emission cavities and the structure constraints for the sample as a whole suggest that these young disks may eventually produce planetary systems, and have perhaps already started.

We present new results from a significant extension of our previous high angular resolution (0. ′′ 3 ≈ 40 AU) Submillimeter Array survey of the 340 GHz (880 µm) thermal continuum emission from dusty circumstellar disks in the ∼1 Myr-old Ophiuchus starforming region. An expanded sample is constructed to probe disk structures that emit significantly lower millimeter luminosities (hence dust masses), down to the median value for T Tauri stars. Using a Monte Carlo radiative transfer code, the millimeter visibilities and broadband spectral energy distribution for each disk are simultaneously reproduced with a two-dimensional parametric model for a viscous accretion disk that has a surface densityWe find wide ranges of characteristic radii (R c = 14-198 AU) and disk masses (M d = 0.004-0.143 M ⊙ ), but a narrow distribution of surface density gradients (γ = 0.4-1.1) that is consistent with a uniform value γ = 0.9 ± 0.2 and independent of mass (or millimeter luminosity). In this sample, we find a correlation between the disk luminosity/mass and characteristic radius, such that fainter disks are both smaller and less massive. We suggest that this relationship is an imprint of the initial conditions inherited by the disks at their formation epoch, compare their angular momenta with those of molecular cloud cores, and speculate on how future observations can help constrain the distribution of viscous evolution timescales. No other correlations between disk and star properties are found. The inferred disk structures are briefly compared with theoretical models for giant planet formation, although resolution limitations do not permit us to directly comment on material inside R ≈ 20 AU. However, there is some compelling evidence for the evolution of dust in the planet formation region: 4/17 disks in the sample show resolved regions of significantly reduced millimeter optical depths within ∼20-40 AU of their central stars.

From the masses of planets orbiting our Sun, and relative elemental abundances, it is estimated that at birth our Solar System required a minimum disk mass of ∼0.01 M within ∼100 AU of the star 1-4 . The main constituent, gaseous molecular hydrogen, does not emit from the disk mass reservoir 5 , so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide 6 . Carbon monoxide emission generally probes the disk surface, while the conversion from dust emission to gas mass requires knowl-1

We present high resolution (0. ′′ 3 = 16 AU), high signal-to-noise ratio Submillimeter Array observations of the 870 µm (345 GHz) continuum and CO J=3−2 line emission from the protoplanetary disk around TW Hya. Using continuum and line radiative transfer calculations, those data and the multiwavelength spectral energy distribution are analyzed together in the context of simple two-dimensional parametric disk structure models. Under the assumptions of a radially invariant dust population and (vertically integrated) gas-to-dust mass ratio, we are unable to simultaneously reproduce the CO and dust observations with model structures that employ either a single, distinct outer boundary or a smooth (exponential) taper at large radii. Instead, we find that the distribution of millimeter-sized dust grains in the TW Hya disk has a relatively sharp edge near 60 AU, contrary to the CO emission (and optical/infrared scattered light) that extends to a much larger radius of at least 215 AU. We discuss some possible explanations for the observed radial distribution of millimeter-sized dust grains and the apparent CO-dust size discrepancy, and suggest that they may be hallmarks of substructure in the dust disk or natural signatures of the growth and radial drift of solids that might be expected for disks around older pre-main sequence stars like TW Hya.