Can grain growth explain transition disks?

TL;DR

This study tests whether grain growth alone can explain the dust depletion observed in transition disks by combining dust evolution models with radiative transfer, finding that grain growth can produce infrared dips but cannot account for large millimeter cavities.

Contribution

It demonstrates that grain growth and transport effects alone are insufficient to explain the large millimeter emission cavities in transition disks.

Findings

Grain growth can create infrared dips in SEDs.

Large grains remain detectable at millimeter wavelengths.

Grain growth alone cannot produce the observed large cavities.

Abstract

Aims: Grain growth has been suggested as one possible explanation for the diminished dust optical depths in the inner regions of protoplanetary "transition" disks. In this work, we directly test this hypothesis in the context of current models of grain growth and transport. Methods: A set of dust evolution models with different disk shapes, masses, turbulence parameters, and drift efficiencies is combined with radiative transfer calculations in order to derive theoretical spectral energy distributions (SEDs) and images. Results: We find that grain growth and transport effects can indeed produce dips in the infrared SED, as typically found in observations of transition disks. Our models achieve the necessary reduction of mass in small dust by producing larger grains, yet not large enough to be fragmenting efficiently. However, this population of large grains is still detectable at millimeter wavelengths. Even if perfect sticking is assumed and radial drift is neglected, a large population of dust grains is left behind because the time scales on which they are swept up by the larger grains are too long. This mechanism thus fails to reproduce the large emission cavities observed in recent millimeter-wave interferometric images of accreting transition disks.