Protostellar Disk Formation Regimes: Angular Momentum Conservation versus Magnetic Braking

TL;DR

This study compares hydrodynamic and non-ideal MHD models to explain the diverse sizes of protostellar disks, finding that different physical processes dominate at various evolutionary stages, aligning well with observed disk size distributions.

Contribution

It introduces a combined theoretical framework using hydrodynamics and non-ideal MHD to explain the observed range of protostellar disk sizes across different mass regimes.

Findings

Synthetic disk radius distribution matches observations

Hydrodynamics dominate at low masses, MHD at higher masses

Explains rarity of large disks and prevalence of small disks

Abstract

Protostellar disks around young protostars exhibit diverse properties, with their radii ranging from less than ten to several hundred au. To investigate the mechanisms shaping this disk radius distribution, we compiled a sample of 27 Class 0 and I single protostars with resolved disks and dynamically determined protostellar masses from the literature. Additionally, we derived the radial profile of the rotational to gravitational energy ratio in dense cores from the observed specific angular momentum profiles in the literature. Using these observed protostellar masses and rotational energy profile, we computed theoretical disk radii from the hydrodynamic and non-ideal magnetohydrodynamic (MHD) models in Lee et al. (2021, 2024) and generated synthetic samples to compare with the observations. In our theoretical model, the disk radii are determined by hydrodynamics when the central protostar+disk mass is low. After the protostars and disks grow and exceed certain masses, the disk radii become regulated by magnetic braking and non-ideal MHD effects. The synthetic disk radius distribution from this model matches well with the observations. This result suggests that hydrodynamics and non-ideal MHD can be dominant in different mass regimes (or evolutionary stages) depending on the rotational energy and protostar+disk mass. This model naturally explains the rarity of large (>100 au) disks and the presence of very small (<10 au) disks. It also predicts that the majority of protostellar disks have radii of a few tens of au, as observed.