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

TL;DR

This study models dust sedimentation in protoplanetary disks to understand its impact on the 10 micron silicate feature, revealing limited effects unless a bimodal grain size distribution is present.

Contribution

It demonstrates that dust sedimentation alone cannot fully explain the observed diversity of silicate features in disks, highlighting the importance of grain size distribution.

Findings

Sedimentation can make flat features more pointy but only under specific conditions.

A bimodal grain size distribution enhances the sedimentation effect on spectral features.

Sedimentation alone does not account for the observed variety of silicate features.

Abstract

Aims: We wish to investigate what the effect of dust sedimentation is on the observed 10 mum feature of protoplanetary disks 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. 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 mum 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 mum silicate features observed in protoplanetary disks.