USING FU ORIONIS OUTBURSTS TO CONSTRAIN SELF-REGULATED PROTOSTELLAR DISK MODELS

One-dimensional, convective, vertical structure models and one-dimensi onal time-dependent, radial diffusion models are combined to create a self-consistent picture in which FU Orionis outbursts occur in young s tellar objects (YSOs) as the result of a large-scale, self-regulated, thermal ionization instability in the surrounding protostellar accreti on disk. Although active accretion disks have long been postulated to be ubiquitous among low-mass YSOs, few constraints have until now been imposed on physical conditions in these disks. By fitting the results of time-dependent disk models to observed timescales of FU Orionis ev ents, we estimate the magnitude of the effective viscous stress in the inner disk (r less than or similar to 1 AU) to be, in accordance with an ad hoc ''alpha'' prescription, the product of the local sound spee d, pressure scale height, and an efficiency factor alpha of 10(-4) whe re hydrogen is neutral and 10(-3) where hydrogen is ionized. We hypoth esize that all YSOs receive infall onto their outer disks which is ste ady (or slowly declining with time) and that FU Orionis outbursts are self-regulated, disk outbursts which occur only in systems which trans port matter inward at a rate sufficiently high to cause hydrogen to be ionized in the inner disk. We estimate a critical mass flux of M(crit ) = 5 x 10(-7) M. yr-1 independent of the magnitude of alpha for syste ms with one solar mass, three solar radius central objects. Infall acc retion rates in the range of M(in) = (1-10) x 10(-6) M. yr-1 produce o bserved FU Orionis timescales consistent with estimates of spherical m olecular cloud core collapse rates. Modeled ionization fronts are typi cally initiated near the inner edge of the disk and propagate out to a disgtance of several tens of stellar radii. Beyond this region, the d isk transports mass steadily inward at the supplied constant infall ra te. Mass flowing through the innermost disk annulus is equal to M(in) only in a time-averaged sense and is regulated by the ionization of hy drogen in the inner disk such that long intervals (almost-equal-to 100 0 yr) of low-mass flux: (1-30) x 10(-8) M. yr-1 are punctuated by shor t intervals (almost-equal-to 100 yr) of high-mass flux: (1-30) x 10(-5 ) M. yr-1. Timescales and mass fluxes derived for quiescent and outbur st stages are consistent with estimates from observations of T Tauri a nd FU Orionis systems, respectively.