One dimensional, convective, vertical structure models and one dimensional, time dependent, radial di usion 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. Although active accretion disks have long been postulated to be ubiquitous among low mass young stellar objects, few constraints have until now been imposed on physical conditions in these disks. By tting the results of time dependent disk models to observed time scales of FU Orionis events, we estimate the magnitude of the e ective viscous stress in the inner disk (r < 1 AU) to be, in accordance with an ad hoc \alpha" prescription, the product of the local sound speed, pressure scale height, and an e ciency factor of 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 su ciently high to cause hydrogen to be ionized in the inner disk. We estimate a critical mass ux of _ M crit = 5 10 7 M = yr independent of the magnitude of for systems with one solar mass, three solar radius central objects. Infall accretion rates in the range of _ M in = (1 10) 10 6 M = yr produce observed FU Orionis time scales consistent with estimates of spherical molecular cloud core collapse rates. Modeled ionization fronts are typically initiated near the inner edge of the disk and propagate out to a distance of 1 Lick Observatory Bulletin No. 2 Send proofs to M. Bivens, UCO/Lick Observatory, University of California, Santa Cruz 95064 { 2 { several tens of stellar radii. Beyond this region, the disk transports mass steadily inward at the supplied constant infall rate. Mass owing through the innermost disk annulus is equal to _ M in only in a time averaged sense and is regulated by the ionization of hydrogen in the inner disk such that long intervals ( 1000 yrs) of low mass ux: (1 30) 10 8 M = yr, are punctuated by short intervals ( 100 yrs) of high mass ux: (1 30) 10 5 M = yr. Time scales and mass uxes derived for quiescent and outburst stages are consistent with estimates from observations of T Tauri and FU Orionis systems respectively.

No abstract

The questions of how planets form and how common Earth-like planets are can be addressed by measuring the distribution of exoplanet masses and orbital periods. We report the occurrence rate of close-in planets (with orbital periods less than 50 days), based on precise Doppler measurements of 166 Sun-like stars. We measured increasing planet occurrence with decreasing planet mass (M). Extrapolation of a power-law mass distribution fitted to our measurements, df/dlogM = 0.39 M(-0.48), predicts that 23% of stars harbor a close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoretical models of planet formation predict a deficit of planets in the domain from 5 to 30 Earth masses and with orbital periods less than 50 days. This region of parameter space is in fact well populated, implying that such models need substantial revision.