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 (0 ''.2-0 ''.5) 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 kappa(nu) Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (beta), we find that beta(R) increases from beta < 0.5 at similar to 20 AU to beta > 1.5 for R greater than or similar to 80 AU, inconsistent with a constant value of beta across the disk (at the 10 sigma level). Furthermore, if radial variations of kappa(nu) 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 greater than or similar to 70 AU) to millimeter- and centimeter-sized grains in the inner disk regions (R less than or similar to 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.