Non-steady Accretion in Protostars

TL;DR

This paper investigates the variability of protostellar disk accretion, proposing that combined MRI and gravitational instabilities cause episodic rapid accretion outbursts, aligning with observations of objects like FU Ori.

Contribution

It demonstrates that MRI and gravitational instabilities together likely trigger episodic accretion outbursts in protostellar disks, explaining observed variability.

Findings

MRI and gravitational instability combine to produce accretion outbursts

Constant alpha disks cannot match infall rates across all radii

Model aligns with observed FU Ori outbursts

Abstract

Observations indicate that mass accretion rates onto low-mass protostars are generally lower than the rates of infall to their disks; this suggests that much of the protostellar mass must be accreted during rare, short outbursts of rapid accretion. We explore when protostellar disk accretion is likely to be highly variable. While constant $\alpha$ disks can in principle adjust their accretion rates to match infall rates, protostellar disks are unlikely to have constant $\alpha$. In particular we show that neither models with angular momentum ransport due solely to the magnetorotational instability (MRI) nor ravitational instability (GI) are likely to transport disk mass at rotostellar infall rates over the large range of radii needed to move infalling envelope material down to the central protostar. We show that the MRI and GI are likely to combine to produce outbursts of rapid accretion starting at a few AU. Our analysis is consistent with the time-dependent models of Armitage, Livio, & Pringle (2001) and agrees with our observational study of the outbursting object FU Ori.