The dynamics of FU Orionis outbursts: Time-dependent accretion through protostellar disks

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Accretion Disks, Protostars, Stellar Mass, Stellar Mass Accretion, Stellar Radiation, Time Dependence, Viscosity, Cygnus Constellation, Ionization, Molecular Clouds, Orion Constellation, Thermal Instability

Scientific paper

Although active accretion disks are thought to be ubiquitous among low mass young stellar objects (YSO's), few constraints are imposed on physical conditions in these disks. Time scales of FU Orionis events (dramatic outbursts thought to occur in all YSO's) are used to estimate the magnitude of the disk's viscosity. We propose that disks which receive steady infall, Min-dot, at a rate greater than the critical value: Mcrit-dot = 5 x 10-7 solar mass/yr for which hydrogen ionizes in the disk, will be subject to thermally triggered, repetitive accretion outbursts. Results from one dimensional convective vertical structure and radial time dependent diffusion calculations create a self-consistent picture in which FU Orionis outbursts occur due to self-regulated ionization fronts propagating radially out to several tenths of an AU. With viscous efficiency alpha between 10-3 and 10-4, disk models with one solar mass central objects accreting (1-10) x 10-6 solar mass/yr (consistent with estimates of molecular cloud core collapse rates) display FU Orionis outburst behavior and time scales. Beyond 1/4 AU modeled disks transport mass steadily inward at the input rate: Min-dot, while inner regions are regulated by the thermal instability such that long intervals (approximately 1000 yrs) of low mass flux: (1-30) x 10-8 solar mass/yr, are punctuated by short intervals (approximately 100 yrs) of high mass flux: (1-30) x 10-5 solar mass/yr. The model is substantiated by black body fits to time dependent models which display spectral features characteristic of observed objects. The rise time of Fuor V1515 Cyg is matched by a self-regulated outburst model. The rapid rise times of FU Ori and V1057 Cyg require the application of modest perturbations. That after peak light in V1057 Cyg short wavelength radiation decayed more rapidly than 2-5 micron radiation is naturally explained if mass flux drops in the inner disk (r less than 1/4 AU) while remaining steady in the outer disk. The decrease in line width seen after peak light in V1057 Cyg may be accounted for by the decreasing Keplerian velocity encountered by the outward propagating ionization front. We suggest that FU Orionis outbursts primarily occur to systems during the embedded phase with ages less than several x105 yrs. Evidence is reviewed which suggests that HL Tau is a system subject to FU Orionis type outbursts which is currently quiescent.

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