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 sub-arcsecond angular resolution ( 0.2 ^ { \prime \prime } -0.5 ^ { \prime \prime } ) 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 \sim 20 AU to \beta > 1.5 for R \gtrsim 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 sub-millimeter-sized grains in the outer disk ( R \gtrsim 70 AU ) to millimeter and centimeter-sized grains in the inner disk regions ( R \lesssim 70 AU ) .
We compare our observational constraint on a _ { max } ( R ) with predictions from physical models of dust evolution in proto-planetary 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 .