Kavli Affiliate: Gregory J. Herczeg
| First 5 Authors: Hanpu Liu, Gregory J. Herczeg, Doug Johnstone, Carlos Contreras-Peña, Jeong-Eun Lee
| Summary:
FU Ori type objects (FUors) are decades-long outbursts of accretion onto
young stars that are strong enough to viscously heat disks so that the disk
outshines the central star. We construct models for FUor objects by calculating
emission components from a steady-state viscous accretion disk, a
passively-heated dusty disk, magnetospheric accretion columns, and the stellar
photosphere. We explore the parameter space of the accretion rate and stellar
mass to investigate implications on the optical and near-infrared spectral
energy distribution and spectral lines. The models are validated by fitting to
multi-wavelength photometry of three confirmed FUor objects, FU Ori, V883 Ori
and HBC 722 and then comparing the predicted spectrum to observed optical and
infrared spectra. The brightness ratio between the viscous disk and the stellar
photosphere, $eta$, provides an important guide for identifying viscous
accretion disks, with $eta=1$ ("transition line") and $eta=5$ ("sufficient
dominance line") marking turning points in diagnostics, evaluated here in the
near-infrared. These turning points indicate the emergence and complete
development of FUor-characteristic strong CO absorption, weak metallic
absorption, the triangular spectral continuum shape in the $H$-band, and
location in color-magnitude diagrams. Lower stellar mass $M_*$ and higher
accretion rate $dot{M}$ lead to larger $eta$; for $M_*=0.3~{rm M_odot}$,
$eta=1$ corresponds to $dot{M}=2times10^{-7}~{rm M_odot}/$yr and $eta=5$
to $dot{M}=6times10^{-7}~{rm M_odot}/$yr. The sufficient dominance line
also coincides with the expected accretion rate where accreting material
directly reaches the star. We discuss implications of the models on extinction
diagnostics, FUor brightening timescales, viscous disks during initial
protostellar growth, and eruptive young stellar objects (YSO) associated with
FUors.
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