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Register a new userImaging the Disk around TW Hya with the Submillimeter Array
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C. Qi
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We present ~2-4 aperture synthesis observations of the circumstellar disk surrounding the nearby young star TW Hya in the CO J=2--1 and J=3--2 lines and associated dust continuum obtained with the partially completed Submillimeter Array. The extent and peak flux of the 230 and 345 GHz dust emission follow closely the predictions of the irradiated accretion disk model of Calvet et al. (2002). The resolved molecular line emission extends to a radius of at least 200 AU, the full extent of the disk visible in scattered light, and shows a clear pattern of Keplerian rotation. Comparison of the images with 2D Monte Carlo models constrains the disk inclination angle to 7+/-1 degrees. The CO emission is optically thick in both lines, and the kinetic temperature in the line formation region is ~20K. Substantial CO depletion, by an order of magnitude or more from canonical dark cloud values, is required to explain the characteristics of the line emission.
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For over a decade, the structure of the inner cavity in the transition disk of TW Hydrae has been a subject of debate. Modeling the disk with data obtained at different wavelengths has led to a variety of proposed disk structures. Rather than being inconsistent, the individual models might point to the different faces of physical processes going on in disks, such as dust growth and planet formation. Our aim is to investigate the structure of the transition disk again and to find to what extent we can reconcile apparent model differences. A large set of high-angular-resolution data was collected from near-infrared to centimeter wavelengths. We investigated the existing disk models and established a new self-consistent radiative-transfer model. A genetic fitting algorithm was used to automatize the parameter fitting. Simple disk models with a vertical inner rim and a radially homogeneous dust composition from small to large grains cannot reproduce the combined data set. Two modifications are applied to this simple disk model: (1) the inner rim is smoothed by exponentially decreasing the surface density in the inner ~3 AU, and (2) the largest grains (>100 um) are concentrated towards the inner disk region. Both properties can be linked to fundamental processes that determine the evolution of protoplanetary disks: the shaping by a possible companion and the different regimes of dust-grain growth, respectively. The full interferometric data set from near-infrared to centimeter wavelengths requires a revision of existing models for the TW Hya disk. We present a new model that incorporates the characteristic structures of previous models but deviates in two key aspects: it does not have a sharp edge at 4 AU, and the surface density of large grains differs from that of smaller grains. This is the first successful radiative-transfer-based model for a full set of interferometric data.
We present Very Large Array observations at 3.5 cm of the nearby young star TW Hya that show the emission is constant in time over weeks, months and years, and spatially resolved with peak brightness temperature ~10 K at ~0.25 (15 AU) resolution. These features are naturally explained if the emission mechanism at this wavelength is thermal emission from dust particles in the disk surrounding the star. To account quantitatively for the observations, we construct a self-consistent accretion disk model that incorporates a population of centimeter size particles that matches the long wavelength spectrum and spatial distribution. A substantial mass fraction of orbiting particles in the TW Hya disk must have agglomerated to centimeter size. These data provide the first clear indication that dust emission from protoplanetary disks may be observed at centimeter wavelengths, and that changes in the spectral slope of the dust emission may be detected, providing constraints on dust evolution and the planet formation process.
The face-on disk around TW Hya is imaged in scattered light at wavelengths of 1.1 and 1.6 micron using the coronagraph in the Near Infrared Camera and Multi Object Spectrometer aboard the Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20-230 AU. The surface brightness declines as a power law of r^(-2.6+/-0.1) between 45 and 150 AU. The scattering profile indicates that the disk is flared, not geometrically flat. The disk, while spatially unresolved in thermal radiation at wavelengths of 12 and 18 micron in observations from the W. M. Keck Observatory, shows amorphous and crystalline silicate emission in its spectrum. A disk with silicate grains of a ~1 micron in size in its surface layers can explain the shape of the mid-infrared spectrum. Much larger grains in the disk interior are necessary to fit the millimeter-wave spectral energy distribution, and hence grain growth from an original interstellar size population may have occurred.
We present molecular line observations of 13CO and C18O J=3-2, CN N = 3 - 2, and CS J=7-6 lines in the protoplanetary disk around TW Hya at a high spatial resolution of ~9 au (angular resolution of 0.15), using the Atacama Large Millimeter/Submillimeter Array. A possible gas gap is found in the deprojected radial intensity profile of the integrated C18O line around a disk radius of ~58 au, slightly beyond the location of the au-scale dust clump at ~52 au, which resembles predictions from hydrodynamic simulations of planet-disk interaction. In addition, we construct models for the physical and chemical structure of the TW Hya disk, taking account of the dust surface density profile obtained from high spatial resolution dust continuum observations. As a result, the observed flat radial profile of the CN line intensities is reproduced due to a high dust-to-gas surface density ratio inside ~20 au. Meanwhile, the CO isotopologue line intensities trace high temperature gas and increase rapidly inside a disk radius of ~30 au. A model with either CO gas depletion or depletion of gas-phase oxygen elemental abundance is required to reproduce the relatively weak CO isotopologue line intensities observed in the outer disk, consistent with previous atomic and molecular line observations towards the TW Hya disk. {Further observations of line emission of carbon-bearing species, such as atomic carbon and HCN, with high spatial resolution would help to better constrain the distribution of elemental carbon abundance in the disk gas.
We report the detection of spiral substructure in both the gas velocity and temperature structure of the disk around TW~Hya, suggestive of planet-disk interactions with an unseen planet. Perturbations from Keplerian rotation tracing out a spiral pattern are observed in the SE of the disk, while significant azimuthal perturbations in the gas temperature are seen in the outer disk, outside 90~au, extending the full azimuth of the disk. The deviation in velocity is either $Delta v_{phi} , / , v_{rm kep} sim 0.1$ or $Delta v_{z} , / , v_{rm kep} sim 0.01$ depending on whether the perturbation is in the rotational or vertical direction, while radial perturbations can be ruled out. Deviations in the gas temperature are $pm 4$ K about the azimuthally averaged profile, equivalent to deviations of $Delta T_{rm gas} , / , T_{rm gas} sim 0.05$. Assuming all three structures can be described by an Archimedean spiral, measurements of the pitch angles of both velocity and temperature spirals show a radially decreasing trend for all three, ranging from 9$^{circ}$ at 70 au, dropping to 3$^{circ}$ at 200 au. Such low pitch-angled spirals are not readily explained through the wake of an embedded planet in the location of previously reported at 94 au, but rather require a launching mechanism which results in much more tightly wound spirals. Molecular emission tracing distinct heights in the disk is required to accurately distinguish between spiral launching mechanisms.
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