VLA Studies of Disks around T Tauri stars David J. Wilner Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA Motivation

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1 VLA Studies of Disks around T Tauri stars David J. Wilner Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA Motivation Much activity is devoted to characterizing the properties of disks around young stars to extract information on the physics of mass accretion and of planet building. Early analysis of the spectral energy distributions of disks indicated outer radii of 10's to 100's of AU (Adams, Lada & Shu 1987) and masses sucient to form Solar Systems like our own (Beckwith et al. 1990). Statistical studies suggest that disks dissipate on timescales of order 10 Myr (Strom et al. 1989), compatible with the standard view of giant planet formation by dust coagulation, planetesimal formation, and core accretion of nebular gas. But many issues remain in this empirical outline of disk evolution. Which disks provide fertile ground for planets? When does dust settle and how do grains grow? How is nebular gas lost? Can we see the signatures of planetary bodies forming within disks, for example tidal gaps and holes? While improved modeling of spatially unresolved observations continues to provide insight (e.g. Chiang et al. 2001), direct imaging of disks at many wavelengths is essential to address these questions. Making resolved images of disks presents challenges. For the large sample of T Tauri stars in nearby dark clouds like Taurus and Chameleon at 140 pc, the 10 AU diameter of a Jupiter orbit subtends only 70 milliarcseconds. In addition, the disk material beyond a few stellar radii is cool, well below 1000 K, and a large fraction of disk emission emerges in the far-infrared, a spectral range dicult to access from the ground and lacking large apertures in space. In the optical and near-infrared, disks may be detected in scattered light, and the necessary resolution is provided by the Hubble Space Telescope and large ground-based telescopes like the ESO VLT employing adaptive optics. At these wavelengths, high contrast with stellar photospheres means that very careful point-spreadfunction subtraction or coronography must be used, and the innermost regions of the disks remain dicult to probe at all. Observations at millimeter wavelengths oer some advantages (see the review by Wilner & Lay 2000) because: (1) the dust emission is (almost) entirely optically thin and probes the full disk volume, so the observed emission is proportional to mass, weighted by temperature; (2) emission from circumstellar dust dominates emission from the stellar photosphere, and contrast is not a problem; (3) the technique of interferometry allows for imaging observations that obtain the necessary high angular resolution. The main problem today with observations at millimeter wavelengths is that very high sensitivity and very good at-

2 2 David J. Wilner mospheric conditions are required to realize imaging observations at the highest resolutions available. 2 Imaging Disks with the Very Large Array The Very Large Array (VLA) of the National Radio Astronomy Observatory in New Mexico, USA, provides two unique capabilities for disk structure studies. First, the angular resolution for imaging thermal emission is much better than can be achieved with any other millimeter array. For the standard \A" conguration, the synthesized beam size at the shortest operating wavelength of 7 mm is about 40 milliarcseconds. Work recently completed on a real-time optical ber connection between the VLA and the Very Long Baseline Array antenna located at Pie Town, New Mexico, eectively doubles the resolution of the VLA while maintaining excellent imaging characteristics, especially for northern sources. The resolution is similar to that expected for VLTI in the thermal infrared. Second, the long millimeter wavelengths accessible to the VLA are especially advantageous at very high angular resolution because dust opacity is low and emission emerges from within the highest surface density regions of the inner disks. To achieve sub-arcsecond imaging with the VLA at 7 mm is not always easy. Phase correction techniques are usually required to overcome the atmospheric seeing. Unfortunately, the disks around young stars are too faint for self-calibration, whereby phase closure relations are determined in a coherence time. Instead, a mode of \fast-switching" phase referencing to nearby calibrators is used that eectively stops the phase uctuations for baselines longer than a few hundred meters. This technique allows for diraction limited imaging at the longest baselines, but generally only during the winter months when the atmosphere is most stable, and especially on winter nights when coherence times are longest (several minutes or more). Since the VLA varies its resolution through repositioning of the array antennas in four basic congurations over a period of approximately 16 months, the \A" conguration required for high resolution imaging is only sometimes available at the appropriate time of year. Dust emission drops steeply toward long wavelengths, and the strength of the 7 mm signal from the disks around T Tauri stars is modest at the highest angular resolution. For a geometrically thin disk, the ux ds from a disk element lling d is ds = B (T )(1?e? ) cos id, where B (T ) is the Planck function, i is the inclination, and the optical depth is given by = = cos i where is the surface density and the is the mass opacity. Following common practice, we adopt a long wavelength opacity with power law form and adopt the normalization advocated by Beckwith et al. (1990), i.e. = 0:1(=10 12 Hz) cm 2 g?1 ; = 1, recognizing that this expression hides many uncertainties associated with grain size, composition, and dust-to-gas ratio. For example, the emissivity must be affected by the evaporation of dierent grain constituents, starting with water ices at T 200 K. For ducial values appropriate to VLA observations, where the best sensitivity in the 7 mm band is obtained at 43.4 GHz, assuming low optical

3 depth and the Rayleigh-Jeans limit T ds = 0:11 mjy 100 K 100 g=cm 2 43:4 GHz VLA Studies of Disks (1) 40 mas where is the synthesized beam Gaussian fwhm size. For the inner parts of disks where the temperature exceeds 100 K and the surface density is high, the VLA can detect the disk material in a single 8 hour track. For sources at 140 pc, this detectable signal corresponds to < 1 Jupiter mass of an interstellar mixture of gas and dust lling the 5 AU beam. This high sensitivity introduces a potential problem since very small amounts of ionized gas are also detectable, and the plasma can contaminate the signal from dust. However, the plasma contribution (if any) can be estimated accurately from longer wavelength VLA data. 3 Two Recent Examples 3.1 TW Hya The TW Hya system is almost three times closer than the T Tauri stars associated with dark clouds (56 7 pc), and we selected this source for VLA imaging despite an extreme southern location that makes it a dicult target. Wilner et al. (2000) provide a more complete description of the TW Hya observations than the brief summary provided here. Figure 1 (lower left) shows the long wavelength spectrum of TW Hya, including our observations from the VLA and the BIMA array. The spectrum shows dust emission far in excess of the stellar photosphere, and, like most T Tauri stars, this excess emission is well tted by a family of thin disk models parameterized by radial power laws in temperature and surface density. In these models, the temperature distribution is determined by the slope of the spectrum at infrared wavelengths where the disk is optically thick. Stellar irradiation together with aring of the outer regions tend to give T (r) r?q, with q 0:5, consistent with the TW Hya spectrum. The solid lines in Figure 1 show spectra derived from a series of face-on disk models, assuming the usual (constant) dust opacity law, with (r) / (r=1 AU)?p and p = 0; 0:5; 1:0, and 1:5 (with the mass of gas+dust adjusted from to M to provide the best least squares t). The masses depend on the millimeter mass opacity and are uncertain, especially if the gas-to-dust ratio has evolved from the standard value. In any case, the spectrum is not very sensitive to the details of the surface density distribution, as is well known. Note that the models underpredict the observed emission at 3.6 cm. It's possible that the 3.6 cm emission arises from hot plasma, either from a stellar wind or pre-main-sequence magnetic activity, though a population of very large dust grains would also provide an explanation. Even at the extreme, an ionized plasma contributes a tiny fraction of the 7 mm emission. Figure 1 (upper left) show images of TW Hya made from the VLA 7 mm data at two dierent resolutions. The disk is clearly visible. These resolved 7 mm

4 4 David J. Wilner images are very sensitive to the central concentration of the disk emission, parameterized by p + q in the power law models. Figure 1 (right) shows images made from a series of disk models that match the TW Hya spectrum for a range of values of p (taking q = 0:5). To account for the spatial ltering of the interferometer observations, the models were imaged from the (u; v) tracks obtained for TW Hya for two resolutions and deconvolved in the standard way. The best tting value of p is near unity. While inhomogeneities are likely present in the disk, and changes in disk composition and opacity will modify the energy balance and structure close to the star, the overall structure of the TW Hya disk appears amenable to this simple power law description, at least for radii outside 3 AU. The surface density distribution is consistent with that obtained from images of near-infrared scattered light (Trilling et al. 2001), for radii beyond the coronographic mask (> 50 AU) to an outer radius of 225 AU. Fig. 1. A summary of results for VLA 7 mm imaging of TW Hya. upper left: Images made from the 7 mm data at 0: 00 6 resolution to emphasize the extended low brightness emission and at 0: 00 1 resolution where only a weak signal at size scale 10 AU remains at the center of the larger structure. lower left: Long wavelength spectrum showing best t power law disk models with values of the surface density power law index of 0.5, 1 and 1.5 (the dotted lines indicate a possible plasma component). right: Simulated VLA images for the three model disks whose spectra are shown. A logarithmic grey scale shows low brightness emission.

5 VLA Studies of Disks DG Tau DG Tauri is a well-studied, at spectrum, classical T Tauri star. The 2.7 mm dust continuum emission from the disk was resolved by Dutrey et al. (1996), who measured a size of about 1 00 and an orientation nearly perpendicular to the optical jet that extends to larger size scales. Figure 2 shows two high resolution VLA 7 mm images of DG Tau. Strong emission is detected from the inner part of the inclined disk. The most interesting aspect of the new 7 mm images is the gross departure from simple power law structure at radii < 10 AU. The highest resolution image, including the VLA- Pie Town link, shows two peaks connected by curved bridges of emission. This structure is presumably related to that seen in near-infrared lunar occultation observations that show a single star with an extended \shell" 455 milliarcseconds fwhm (6 AU), which accounts for about 25% of the emission (Leinert et al. 1991). What explains the DG Tau morphology? At this high angular resolution, we can no longer precisely locate the star. Does one peak mark circumstellar dust heated from within by the star? If so, what produces the asymmetry? Does an emission peak indicate a condensing protoplanet? Or is the star located between the peaks, heating both of them? If so, then perhaps we are seeing the opening of some kind of gap, perhaps due to the dynamical eect of a protoplanet. Another explanation for the secondary peak might be an ionized blob from the jet; at these low ux levels, such a small, partly optically thick knot would remain very dicult to isolate in observations made at longer wavelengths. We hope to distinguish among these possibilities with a second epoch observation. The timescales for either orbital motion at 5 AU radius, or bipolar outow at 10's of km s?1, are both suciently short that a second image within a few years should show signicant secular changes. References 1. Adams, F.C., Lada, C.J. & Shu, F.H. 1987, ApJ, 312, Beckwith, S.V.W., Sargent, A.I., Chini, R. & Gusten, R. 1990, AJ, 99, Chiang, E.I., Joung, M.K., Creech-Eakman, M.J., Qi, C., Kessler, J.E., Blake, G.A. & van Dishoeck, E.F. 2001, ApJ, 547, Dutrey, A., Guilloteau, S., Duvert, G. Prato, L., Simon, M., Schuster, K. & Menard, F. 1996, A&A, 309, Leinert, C., Haas, M., Richichi, A., Zinnecker, H. & Mundt, R. 1991, A&A, 250, Strom, K.M., Strom, S.E., Edwards, S., Cabrit, S. & Skrutskie, M.F. 1989, AJ, 97, Trilling, D.E., Koerner, D.W., Barnes, J.W., Ftaclas, C., & Brown, R.H. 2001, ApJ, 552, L Wilner, D.J. & Lay, O.P. 2000, in Protostars and Planets IV, eds. V. Mannings, A. Boss and S. Russell, p Wilner, D.J., Ho, P.T.P., Kastner, J.H. & Rodriguez, L.F. 2000, ApJ, 534, L101

6 6 David J. Wilner Fig. 2. High resolution VLA 7 mm images of the DG Tauri system. upper: The inner part of the dust disk; the arrow indicates the position angle of the optical jet observed at larger scales. The ellipses to the right show the orientation of the disk, and the spacings of the ellipses correspond to the orbits of the giant planet orbits in our Solar System. Contours levels are (2; 3; :::) 0:12 mjy. lower A higher resolution image made to emphasize the small scale, high brightness structure. The synthesized beam size is 30 milliarcseconds. The origin of the asymmetric structure is unclear, but either orbital motions or outow should be detectable in a second epoch observation. Acknowledgements I wish to thank the NRAO sta and all of my collaborators on the VLA 7 mm disk work, especially Luis Rodriguez and Paul Ho.