Testing the theory of grain growth and fragmentation by millimeter observations of protoplanetary disks

TL;DR

This study models grain growth and fragmentation in protoplanetary disks to predict their millimeter spectral slopes, comparing these predictions with observations to understand dust evolution and disk mass discrepancies.

Contribution

First physical model predictions of millimeter spectral slopes based on grain growth and fragmentation processes in disks.

Findings

Models reproduce observed mm spectral slopes.

Matching observed flux levels requires dust mass reduction.

Dust reduction could be due to radial drift or early evolution.

Abstract

Context. Observations at sub-millimeter and mm wavelengths will in the near future be able to resolve the radial dependence of the mm spectral slope in circumstellar disks with a resolution of around a few AU at the distance of the closest star-forming regions. Aims. We aim to constrain physical models of grain growth and fragmentation by a large sample of (sub-)mm observations of disks around pre-main sequence stars in the Taurus-Auriga and Ophiuchus star-forming regions. Methods. State-of-the-art coagulation/fragmentation and disk-structure codes are coupled to produce steady-state grain size distributions and to predict the spectral slopes at (sub-)mm wavelengths. Results. This work presents the first calculations predicting the mm spectral slope based on a physical model of grain growth. Our models can quite naturally reproduce the observed mm-slopes, but a simultaneous match to the observed range of flux levels can only be reached by a reduction of the dust mass by a factor of a few up to about 30 while keeping the gas mass of the disk the same. This dust reduction can either be due to radial drift at a reduced rate or during an earlier evolutionary time (otherwise the predicted fluxes would become too low) or due to efficient conversion of dust into larger, unseen bodies.