2007ApJ...669..483Z

We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ∼4000 Å to ∼8 µm. Using an estimated visual extinction A_V∼1.5, a steady disk model with a central star mass ∼0.3 M_☉, and a mass accretion rate ∼2x10^–4 M_☉/yr, we can reproduce the SED of FU Ori quite well. Higher values of extinction used in previous analysis (A_V∼2.1) result in SEDs which are less well fitted by a steady disk model, but might be explained by extra energy dissipation of the boundary layer in the inner disk. With the mid-infrared spectrum obtained by the IRS on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ∼1 AU, using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ∼0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature T_c≥1000 K out to ∼0.5-1 AU, suggesting that the magnetorotational instability can be supported out to that distance. Assuming that the ∼100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter to be α∼0.2-0.02 in the outburst state, consistent with numerical simulations of the magnetorotational instability in disks. The radial extent of the high-M{dot} region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational and magnetorotational instabilities.