Circumstellar Disk Studies with the SMA
 

Disk Structure

Protoplanetary disk structure affects what kinds of planets can be formed, and whether or not planets can be formed at all. The SMA has already gone through two generations of mini-surveys of submillimeter dust emission from ~20 disks, the first at 1 to 2 arcsecond resolution (Andrews & Williams 2007), and the second making use of the longest baselines to provide high quality data on angular scales to <0.3 arcsecond (corresponding to radial scales of <20 AU; Andrews et al. 2009). These studies have employed increasingly sophisticated radiative transfer modeling to extract disk structure parameters. The longest baselines have allowed for the first time direct measurements of the mass content in the regions of disks where planets form. The disk surface densities are found to be comparable to those thought to be present in the early Solar System, and the disk density distributions are consistent with standard accretion disk models. Most remarkable, these observations have revealed large (>20 AU diameter) cavities in the centers of several disks. The cavities are likely dynamically sculpted by unseen planets.




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Figure 1: Comparison of the disk structure model fits and the data. The left panels show the SMA continuum image, corresponding disk model, and residuals (datamodel) as described in figure one of Andrews et al. 2009. Crosshairs mark the disk centers and major axis position angle; their relative lengths represent the disk inclination. The right panels show the broadband SEDs and deprojected visibility profiles (see Section 4.1 of paper for details), with best-fit models overlaid in red. The SED contributions from the stellar photosphere are shown as blue dashed curves (Andrews et al. 2009).


Disk Identification


The SMA has opened up a new region of the sky at southern declinations, which has enabled the first resolved observations of several of the most nearby, isolated circumstellar disks. Spectral line imaging provides details of the disk kinematics, in particular Keplerian rotation and turbulence, as well as constraints on molecular gas excitation, vertical temperature structure, the decoupling of gas and dust, and evidence for gas dispersal associated with the formation of giant planets. Most important are studies of the disk around TW Hya (Qi et al. 2004, 2006, 2008), at a distance of only 50 pc, the closest gas-rich disk (and almost three times closer than the large sample of disks associated with nearby dark clouds). Other impressive examples of nearby disks newly imaged by the SMA include HD 169142, IM Lup, V4046 Sgr, and 49 Cet (Raman et al. 2006; Panic et al. 2009; Hughes et al. 2008). These nearby disks will be the subject of detailed study for years to come.




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Figure 2: Top: HCO+, DCO+, HCN, and DCN J = 3-2 spectra at the peak continuum (stellar) position. The fluxes of HCO+ and DCO+ are averaged over the beam of DCO+ J = 3-2 (2.6 x 1.6, P.A. = 2.87°). The fluxes of HCN and DCN are averaged over the beam of DCN J = 3-2 (5.9 x 3.2, P.A. = -1.5°). The vertical dotted lines indicate the positions of the fitted VLSR for each molecular transition, except for DCN 3-2, where we adopt the VLSR from that of HCN J = 3-2. Bottom: Velocity channel maps of the HCO+, DCO+, HCN, and DCN J = 3-2 emission toward TW Hydrae. The angular resolutions are 1.6 x 1.1 at P.O. = -6.3° for HCO+ J = 3-2 and 1.6 x 1.1 at P.A. = -0.5° for HCN J = 3-2. The cross indicates the continuum (stellar) position. The axes are offsets from the pointing center in arcseconds. The 1σ contour steps are 0.4, 0.12, 0.35, and 0.09 Jy beam-1 for HCO+, DCO+, HCN, and DCN J = 3-2, respectively, and start at 2σ (Qi et al. 2008).



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