On the Mass Budget Problem of Protoplanetary Disks: Streaming Instability and Optically Thick Emission

TL;DR

This study explores how streaming instability causes optically thick regions in protoplanetary disks, leading to underestimation of disk mass in observations, and suggests this effect explains the missing mass problem.

Contribution

It demonstrates that streaming instability induces optically thick filaments, causing observational mass underestimation, which may resolve the missing mass problem in protoplanetary disks.

Findings

Overdense filaments from streaming instability are optically thick at millimeter wavelengths.

Optically thick regions lead to mass underestimates by factors of 2-7.

Simulation-based radiative transfer shows observational biases in disk mass estimates.

Abstract

Statistical studies of protoplanetary disks and exoplanet populations often exhibit a "missing mass" problem, where observed dust masses in (sub-)millimeter surveys are significantly lower than expected when compared to the mass of evolved exoplanetary systems. We investigate how the streaming instability and subsequent planetesimal formation in protoplanetary disks might solve this missing mass problem when (sub-)millimeter observations are interpreted under the assumption of optically thin emission. We conduct hydrodynamical simulations of the streaming instability with self-gravity after which radiative transfer calculations with dust scattering are performed to measure the (sub-)millimeter intensity. The measured intensity is then used to estimate the disk mass under the assumption of optically thin emission and compared to the true mass in the simulation to calculate the observational bias via the mass excess. We find that the emission from overdense filaments that emerge due to the streaming instability are optically thick at (sub-)millimeter wavelengths, leading to mass excess factors of $\sim 2-7$, even when the optically thick fraction is low.