Protoplanetary Disks from Bondi-Hoyle Accretion
Padoan et al. 2025, Nature Astronomy
(Press release at Barcelona and Dartmouth)
Protoplanetary disks are usually described as finite mass reservoirs left over by the gravitational collapse of the protostar, an assumption that strongly constrains both disk evolution and planet formation models.
We propose a different scenario where protoplanetary disks of pre-main sequence stars are assembled primarily 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, and we predict the dependence of the disk specific angular momentum on the stellar mass.
Our results are based on a new analytical derivation of the scaling of the angular momentum in a turbulent flow, which we also confirm with a numerical simulation of supersonic turbulence.
Example of a PMS bound triple system (bright white dots) from the simulation. As the stars orbit around each other, their long BH tails twist around each other. The Keplerian disks of the stars are not visible because they are too small to be resolved in the simulation.
Time Evolution of Angular Momentum of Class II Disks
Pelkonen et al. 2025, A&A 694, 327
We studied the mass and angular momentum of the accreting gas using passively advected tracer particles in the simulation, and we carried out radiative transfer calculations of near-infrared scattering to generate synthetic JWST observations of Bondi-Hoyle trails of PMS stars.
Gas accreting on class II PMS stars approximately 1 Myr after their formation has enough mass and angular momentum to strongly affect the evolution of the preexisting disks. The accreted angular momentum is large enough to also explain the observed size of class II disks. The orientation of the angular momentum vector can differ significantly from that of the previously accreted gas, which may result in a significant disk warping or misalignment. We also predict that JWST observations of class II stars will be able to detect Bondi-Hoyle trails with a high success rate.
Four example stars (one per row) showing the accretion of the last 10% of their mass on the left (the top x-axis is the mass fraction and the bottom x-axis is the corresponding age of the star), and their late-accretion flows on the right. Left column: Angle ∆φ (dashed blue line, "disk-to-disk" misalignment), angle δφ (dotted blue line, "tracers-to-disk" dispersion), specific angular momentum (solid red line), and median accretion rate (dash-dotted red line). Right column: Column density images in a 6400 au box.
Mass accretion rates as a function of stellar mass. Red diamonds are Class II YSOs in Chamaeleon I, Lupus, and L1688 from Testi et al. (2022), and in Taurus from Gangi et al. (2022). Blue points are large- scale infall rates of star particles from the MHD simulation with local gas density lower than 5 × 104 cm−3 . The dashed and dotted black lines are the expected chromospheric noise levels (Manara et al. 2013, 2017) using the 3 Myr isochrones of Baraffe et al. (2015) and Feiden (2016), respectively.
Column density maps of the simulation. The sink particles are shown with star symbols.