The majority of protostellar disks might be way more massive than we thought: new insights from theory and observation

The properties of protostellar (Class 0/I) disks play an important role in planet formation by setting the environment for dust grain growth and the initial conditions for subsequent protoplanetary (Class II) disk evolution. However, obtaining these properties, especially the disk mass, has been challenging because translating observations of dust thermal emission to disk mass are subject to large uncertainties in dust optical depth and dust temperature. We present new estimates on the mass of typical protostellar disks through a synergy between theory and observation. Using a radiative non-ideal MHD simulation that self-consistently traces the formation of a protostellar disk together with a set of analytic arguments, we argue that a typical protostellar disk should be gravitationally self-regulated, where a balance between infall from the envelope and accretion driven by gravito-viscous angular momentum transport keeps the disk at a marginally gravitationally unstable state with Toomre Q parameter of order unity. Following this physical picture, we formulate a simple model of gravitationally self-regulated disk and use it to fit multi-wavelength dust continuum observations in the VANDAM Orion survey. We find that the majority of observed disks can be fit well with this model. Moreover, the data are better fit by disk models assuming order-unity Q than those assuming larger Q values, suggesting that gravitational instability might be common among protostellar disks. We use this model to produce new estimates of disk properties, and find that typical protostellar disks are significantly more massive than previously expected, with typical disk-to-star mass ratio ~ 1. Such high disk mass may have important implications for planet formation.