Testing particle trapping in transition disks with ALMA^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: pinilla@strw.leidenuniv.nl
² National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Haidian District, 100871 Beijing, PR China
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 2 June 2015
Accepted: 8 September 2015

Abstract

Some protoplanetary disks show evidence of inner dust cavities. Recent observations of gas and dust of these so-called transition disks support the hypothesis that these cavities originate from particle trapping in pressure bumps. We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations at 336 GHz of two transition disks, SR21 and HD 135344B. In combination with previous ALMA observations from Cycle 0 at 689 GHz, we compare the visibility profiles at the two frequencies and calculate the spectral index (α_mm). The observations of SR 21 show a clear shift in the visibility nulls, indicating radial variations of the inner edge of the cavity at the two wavelengths. Notable radial variations of the spectral index are also detected for SR 21 with values of α_mm ~ 3.8–4.2 in the inner region (r ≲ 35 AU) and α_mm ~ 2.6–3.0 outside. An axisymmetric ring (which we call the ring model) or a ring with the addition of an azimuthal Gaussian profile, for mimicking a vortex structure (which we call the vortex model), is assumed for fitting the disk morphology. For SR 21, the ring model better fits the emission at 336 GHz, conversely the vortex model better fits the 689 GHz emission. For HD 135344B, neither a significant shift in the null of the visibilities nor radial variations of α_mm are detected. Furthermore, for HD 135344B, the vortex model fits both frequencies better than the ring model. However, the azimuthal extent of the vortex increases with wavelength, contrary to model predictions for particle trapping by anticyclonic vortices. For both disks, the azimuthal variations of α_mm remain uncertain to confirm azimuthal trapping. The comparison of the current data with a generic model of dust evolution that includes planet-disk interaction suggests that particles in the outer disk of SR 21 have grown to millimetre sizes and have accumulated in a radial pressure bump, whereas with the current resolution there is not clear evidence of radial trapping in HD 135344B, although it cannot be excluded either.