Size-sorting dust grains in the surface layers of protoplanetary disks

Abstract

Context. The shape of dust emission features measured from protoplanetary disks contains information about the typical size of the dust particles residing in these disks. A flattened 10 μm silicate feature is often interpreted as proof that grain growth has taken place, while a pointy feature is taken as evidence for the pristine nature of the dust. Aims. We wish to investigate what the effect of dust sedimentation is on the observed 10 μm feature and how this may affect the interpretation of the observations. Methods. Using a combination of modeling tools, we simulated the sedimentation of a dust grain size distribution in an axisymmetric 2-D model of a turbulent protoplanetary disk, and we used a radiative transfer program to compute the resulting spectra. Results. We find that the sedimentation can turn a flat feature into a pointy one, but only to a limited degree and for a very limited set of particle size distributions. If the distribution is too strongly dominated by small grains, then the feature is pointy even before sedimentation. If the distribution is too strongly dominated by big grains, the sedimentation will not be enough to cause the feature to be pointy. Only if we have a bimodal size distribution, i.e. a very small grain population and a bigger grain population, do we find that the transformation from a flat to a pointy feature upon dust sedimentation is strong. However, our model shows that, if sedimentation is the sole reason for the variety of silicate feature strengths observed in protoplanetary disks, then we would expect to find a correlation such that disks with weak mid- to far-infrared excess have a stronger 10 μm silicate feature than disks with a strong mid- to far-infrared excess. If this is contrary to what is observed, then this would indicate that sedimentation cannot be the main reason for the variety of 10 μm silicate features observed in protoplanetary disks.