The hot inner disk of FU Orionis

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 similar to 4000 angstrom to similar to 8 mu m. Using an estimated visual extinction A(V) similar to 1.5, a steady disk model with a central star mass similar to 0.3M circle dot, and amass accretion rate similar to 2 x 10(-4) M circle dot yr(-1), we can reproduce the SED of FU Ori quite well. Higher values of extinction used in previous analysis ( A(V) similar to 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 similar to 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 similar 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 similar to 0.5 - 1 AU, suggesting that the magnetorotational instability can be supported out to that distance. Assuming that the similar to 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 alpha similar to 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 region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational and magnetorotational instabilities.