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FU Ori type objects (FUors) are decades-long outbursts of accretion ontoyoung stars that are strong enough to viscously heat disks so that the diskoutshines the central star. We construct models for FUor objects by calculatingemission components from a steady-state viscous accretion disk, apassively-heated dusty disk, magnetospheric accretion columns, and the stellarphotosphere. We explore the parameter space of the accretion rate and stellarmass to investigate implications on the optical and near-infrared spectralenergy distribution and spectral lines. The models are validated by fitting tomulti-wavelength photometry of three confirmed FUor objects, FU Ori, V883 Oriand HBC 722 and then comparing the predicted spectrum to observed optical andinfrared spectra. The brightness ratio between the viscous disk and the stellarphotosphere, $\eta$, provides an important guide for identifying viscousaccretion disks, with $\eta=1$ ("transition line") and $\eta=5$ ("sufficientdominance line") marking turning points in diagnostics, evaluated here in thenear-infrared. These turning points indicate the emergence and completedevelopment of FUor-characteristic strong CO absorption, weak metallicabsorption, the triangular spectral continuum shape in the $H$-band, andlocation in color-magnitude diagrams. Lower stellar mass $M_*$ and higheraccretion rate $\dot{M}$ lead to larger $\eta$; for $M_*=0.3~{\rm M_\odot}$,$\eta=1$ corresponds to $\dot{M}=2\times10^{-7}~{\rm M_\odot}/$yr and $\eta=5$to $\dot{M}=6\times10^{-7}~{\rm M_\odot}/$yr. The sufficient dominance linealso coincides with the expected accretion rate where accreting materialdirectly reaches the star. We discuss implications of the models on extinctiondiagnostics, FUor brightening timescales, viscous disks during initialprotostellar growth, and eruptive young stellar objects (YSO) associated withFUors.

Diagnosing FUor-like Sources: The Parameter Space of Viscously Heated Disks in the Optical and Near-IR