Constraints on the Radial Variation of Grain Growth in the AS 209 Circumstellar Disk

We present dust continuum observations of the protoplanetary disk surrounding the pre-main-sequence star AS 209, spanning more than an order of magnitude in wavelength from 0.88 to 9.8 mm. The disk was observed with subarcsecond angular resolution (0farcs2-0farcs5) to investigate radial variations in its dust properties. At longer wavelengths, the disk emission structure is notably more compact, providing model-independent evidence for changes in the grain properties across the disk. We find that physical models which reproduce the disk emission require a radial dependence of the dust opacity κν. Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (β), we find that β(R) increases from β < 0.5 at ~20 AU to β > 1.5 for R >~ 80 AU, inconsistent with a constant value of β across the disk (at the 10σ level). Furthermore, if radial variations of κν are caused by particle growth, we find that the maximum size of the particle-size distribution (a max) increases from submillimeter-sized grains in the outer disk (R >~ 70 AU) to millimeter- and centimeter-sized grains in the inner disk regions (R <~ 70 AU). We compare our observational constraint on a max(R) with predictions from physical models of dust evolution in protoplanetary disks. For the dust composition and particle-size distribution investigated here, our observational constraints on a max(R) are consistent with models where the maximum grain size is limited by radial drift.