Interferometric Upper Limits on Millimeter Polarization of the Disks around DG Tau, GM Aur, and MWC 480

Millimeter-wavelength polarization measurements offer a promising method for probing the geometry of magnetic fields in circumstellar disks. Single dish observations and theoretical work have hinted that magnetic field geometries might be predominantly toroidal, and that disks should exhibit millimeter polarization fractions of 2-3%. While subsequent work has not confirmed these high polarization fractions, either the wavelength of observation or the target sources differed from the original observations. Here we present new polarimetric observations of three nearby circumstellar disks at 2 resolution with the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter Astronomy (CARMA). We reobserve GM Aur and DG Tau, the systems in which millimeter polarization detections have been claimed. Despite higher resolution and sensitivity at wavelengths similar to the previous observations, the new observations do not show significant polarization. We also add observations of a new HAeBe system, MWC 480. These observations demonstrate that a very low (<0.5%) polarization fraction is probably common at large (>100 AU) scales in bright circumstellar disks. We suggest that high-resolution observations may be worthwhile to probe magnetic field structure on linear distances smaller than the disk scale height, as well as in regions closer to the star that may have larger MRI-induced magnetic field strengths.

Confirming the Primarily Smooth Structure of the Vega Debris Disk at Millimeter Wavelengths

Clumpy structure in the debris disk around Vega has been previously reported at millimeter wavelengths and attributed to concentrations of dust grains trapped in resonances with an unseen planet. However, recent imaging at similar wavelengths with higher sensitivity has disputed the observed structure. We present three new millimeter-wavelength observations that help to resolve the puzzling and contradictory observations. We have observed the Vega system with the Submillimeter Array (SMA) at a wavelength of 880 um and angular resolution of 5; with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at a wavelength of 1.3 mm and angular resolution of 5; and with the Green Bank Telescope (GBT) at a wavelength of 3.3 mm and angular resolution of 10. Despite high sensitivity and short baselines, we do not detect the Vega debris disk in either of the interferometric data sets (SMA and CARMA), which should be sensitive at high significance to clumpy structure based on previously reported observations. We obtain a marginal (3-sigma) detection of disk emission in the GBT data; the spatial distribution of the emission is not well constrained. We analyze the observations in the context of several different models, demonstrating that the observations are consistent with a smooth, broad, axisymmetric disk with inner radius 20-100 AU and width >50 AU. The interferometric data require that at least half of the 860 um emission detected by previous single-dish observations with the James Clerk Maxwell Telescope be distributed axisymmetrically, ruling out strong contributions from flux concentrations on spatial scales of <100 AU. These observations support recent results from the Plateau de Bure Interferometer indicating that previous detections of clumpy structure in the Vega debris disk were spurious.

Resolved Submillimeter Observations of the HR 8799 and HD 107146 Debris Disks

We present 880 um Submillimeter Array observations of the debris disks around the young solar analogue HD 107146 and the multiple-planet host star HR 8799, at an angular resolution of 3 and 6, respectively. We spatially resolve the inner edge of the disk around HR 8799 for the first time. While the data are not sensitive enough (with rms noise of 1 mJy) to constrain the system geometry, we demonstrate that a model by Su et al. (2009) based on the spectral energy distribution (SED) with an inner radius of 150 AU predicts well the spatially resolved data. Furthermore, by modeling simultaneously the SED and visibilities, we demonstrate that the dust is distributed in a broad (of order 100 AU) annulus rather than a narrow ring. We also model the observed SED and visibilities for the HD 107146 debris disk and generate a model of the dust emission that extends in a broad band between 50 and 170 AU from the star. We perform an a posteriori comparison with existing 1.3 mm CARMA observations and demonstrate that a smooth, axisymmetric model reproduces well all of the available millimeter-wavelength data.

Empirical Constraints on Turbulence in Protoplanetary Accretion Disks

We present arcsecond-scale Submillimeter Array observations of the CO(3-2) line emission from the disks around the young stars HD 163296 and TW Hya at a spectral resolution of 44 m/s. These observations probe below the ~100 m/s turbulent linewidth inferred from lower-resolution observations, and allow us to place constraints on the turbulent linewidth in the disk atmospheres. We reproduce the observed CO(3-2) emission using two physical models of disk structure: (1) a power-law temperature distribution with a tapered density distribution following a simple functional form for an evolving accretion disk, and (2) the radiative transfer models developed by DAlessio et al. that can reproduce the dust emission probed by the spectral energy distribution. Both types of models yield a low upper limit on the turbulent linewidth (Doppler b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma) detection of a ~300 m/s turbulent linewidth in the upper layers of the HD 163296 disk. These correspond to roughly <10% and 40% of the sound speed at size scales commensurate with the resolution of the data. The derived linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and provide observational support for theoretical predictions of subsonic turbulence in protoplanetary accretion disks.