Coagulation of small grains in disks: the influence of residual infall and initial small-grain content

TL;DR

This paper investigates how residual infall and initial small-grain content influence the presence of small dust grains in protoplanetary disks, challenging the idea that fragmentation alone explains observed dust populations.

Contribution

It explores the potential role of low-level residual infall in maintaining small grain populations in disks, providing an alternative to fragmentation as the primary replenishing mechanism.

Findings

Residual infall can sustain small grains at optically thick levels

Infall rates as low as 10^{-11} Msun/yr are sufficient

Fragmentation remains the most promising process for old disks

Abstract

Turbulent coagulation in protoplanetary disks is known to operate on timescale far shorter than the lifetime of the disk. In the absence of mechanisms that replenish the small dust grain population, protoplanetary disks would rapidly lose their continuum opacity-bearing dust. This is inconsistent with infrared observations of disks around T Tauri stars and Herbig Ae/Be stars, which are usually optically thick at visual wavelengths and show signatures of small (a<~ 3um) grains. A plausible replenishing mechanism of small grains is collisional fragmentation or erosion of large dust aggregates, which model calculations predict to play an important role in protoplanetary disks. If optically thick disks are to be seen as proof for ongoing fragmentation or erosion, then alternative explanations for the existence of optically thick disks must be studied carefully. In this study we explore two scenarios. First, we study the effect of residual, low-level infall of matter onto the disk surface. We find that infall rates as low as 10^{-11} Msun/yr can, in principle, replenish the small grain population to a level that keeps the disk marginally optically thick. However, it remains to be seen if the assumption of such inflow is realistic for star+disk systems at the age of several Myrs, at which winds and jets are expected to have removed any residual envelope. In summary, fragmentation or erosion still appear to be the most promising processes to explain the abundant presence of small grains in old disks.