CONSTRAINTS ON THE RADIAL VARIATION OF GRAIN GROWTH IN THE AS 209 CIRCUMSTELLAR DISK

Abstract

We present dust continuum observations of the protoplanetary disk surrounding\nthe pre-main sequence star AS 209, spanning more than an order of magnitude in\nwavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond\nangular resolution (0.2"-0.5") to investigate radial variations in its dust\nproperties. At longer wavelengths, the disk emission structure is notably more\ncompact, providing model-independent evidence for changes in the grain\nproperties across the disk. We find that physical models which reproduce the\ndisk emission require a radial dependence of the dust opacity \\kappa_{\\nu}.\nAssuming that the observed wavelength-dependent structure can be attributed to\nradial variations in the dust opacity spectral index (\\beta), we find that\n\\beta(R) increases from \\beta<0.5 at \\sim20 AU to \\beta>1.5 for R>80 AU,\ninconsistent with a constant value of \\beta\\ across the disk (at the 10\\sigma\\\nlevel). Furthermore, if radial variations of \\kappa_{\\nu} are caused by\nparticle growth, we find that the maximum size of the particle-size\ndistribution (a_{max}) increases from sub-millimeter-sized grains in the outer\ndisk (R>70 AU) to millimeter and centimeter-sized grains in the inner disk\nregions (R< 70 AU). We compare our observational constraint on a_{max}(R) with\npredictions from physical models of dust evolution in proto-planetary disks.\nFor the dust composition and particle-size distribution investigated here, our\nobservational constraints on a_{max}(R) are consistent with models where the\nmaximum grain size is limited by radial drift.\n