Using FU Orionis Outbursts to Constrain Self-Regulated Protostellar Disk Models

TL;DR

This paper models FU Orionis outbursts as resulting from a self-regulated thermal ionization instability in protostellar disks, linking observed outburst timescales to disk viscosity and infall rates.

Contribution

It combines vertical and radial disk models to constrain the viscosity parameter and critical mass flux for FU Orionis outbursts, providing a self-consistent theoretical framework.

Findings

Effective viscosity parameter estimated as 10^{-4} to 10^{-3}.

Critical mass flux for outbursts is approximately 5x10^{-7} solar masses/year.

Infall rates of (1-10)x10^{-6} solar masses/year match observed outburst timescales.

Abstract

One dimensional, convective, vertical structure models and one dimensional, time dependent, radial diffusion models are combined to create a self-consistent picture in which FU~Orionis outbursts occur in young stellar objects (YSOs) as the result of a large scale, self-regulated, thermal ionization instability in the surrounding protostellar accretion disk. By fitting the results of time dependent disk models to observed time scales of FU~Orionis events, we estimate the magnitude of the effeciency of the effective viscous stress in the inner disk (r < 1 AU) to be, in accordance with the ad hoc ``alpha'' prescription, 10^{-4} where hydrogen is neutral and 10^{-3} where hydrogen is ionized. We hypothesize that all YSOs receive infall onto their outer disks which is steady (or slowly declining with time) and that FU~Orionis outbursts are self-regulated, disk outbursts which occur only in systems which transport matter inward at a rate sufficiently high to cause hydrogen to be ionized in the inner disk. We estimate a critical mass flux of 5x10^{-7} solar masses / year {\it independent of the magnitude of alpha} for systems with one solar mass, three solar radius central objects. Infall accretion rates in the range of (1-10)x10^{-6} solar masses per year produce observed FU~Orionis time scales consistent with estimates of spherical molecular cloud core collapse rates.