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

Abstract Statistical studies of protoplanetary disks and exoplanet populations often exhibit a “missing mass” problem, where the observed dust masses in (sub)millimeter surveys are significantly lower than expected when compared to the masses of 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 usually 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 emerges due to the streaming instability is optically thick at (sub)millimeter wavelengths, leading to mass excess factors of ∼2–7.