The Formation of Protoplanetary Disks through Pre-Main Sequence Bondi-Hoyle Accretion

TL;DR

This paper proposes that protoplanetary disks are primarily formed through Bondi-Hoyle accretion from the parent gas cloud, which supplies both mass and angular momentum, challenging the traditional collapse model.

Contribution

It introduces a new analytical model and numerical simulations showing Bondi-Hoyle accretion as the main mechanism for disk formation and angular momentum acquisition.

Findings

Bondi-Hoyle accretion can supply sufficient mass and angular momentum.

Density fluctuations in turbulence significantly increase angular momentum at disk scales.

Revises current models of disk and planet formation based on turbulence effects.

Abstract

Protoplanetary disks are traditionally described as finite mass reservoirs left over by the gravitational collapse of the protostellar core, a view that strongly constrains both disk evolution and planet formation models. We propose a different scenario where protoplanetary disks of pre-main sequence stars are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. We demonstrate that Bondi-Hoyle accretion can supply not only the mass, but also the angular momentum necessary to explain the observed size of protoplanetary disks. Additionally, we predict how the specific angular momentum of protoplanetary disks scales with stellar mass. Our conclusions are based on a new analytical derivation of the scaling of the angular momentum in turbulent flows, which we confirm with a numerical simulation of supersonic turbulence. A key outcome of our analysis is the recognition that density fluctuations in supersonic turbulence--previously overlooked in studies of cloud and core rotation--lead to a significant increase in angular momentum at disk-forming scales. This revised understanding of disk formation and evolution alleviates several longstanding observational discrepancies and compels substantial revisions to current models of disk and planet formation.