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

TL;DR

This study uses multi-wavelength high-resolution observations of the AS 209 disk to reveal radial variations in dust grain sizes and properties, providing evidence for dust evolution processes like radial drift.

Contribution

It presents the first detailed observational evidence of radial variation in dust grain sizes and opacity spectral index in the AS 209 disk, supporting models of dust growth limited by radial drift.

Findings

eta(R) increases from <0.5 at 20 AU to >1.5 beyond 80 AU

Maximum grain size grows from sub-millimeter in outer regions to millimeter/centimeter in inner regions

Observations are consistent with dust evolution models limited by radial drift

Abstract

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"-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 \sim20 AU to \beta>1.5 for R>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>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 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.