  The emission of FU Orionis objects in outburst has been identified as arising
in rapidly accreting protoplanetary disks, based on a number of observational
properties. A fundamental test of the accretion disk scenario is that the
differentially rotating disk spectrum should produce a variation of rotational
velocity with the wavelength of observation, as spectra taken at longer
wavelengths probe outer, more slowly rotating disk regions. Previous
observations of FU Ori have shown smaller rotation at near-infrared (~ 2.2
micron) wavelengths than observed at optical (~ 0.6 micron) wavelengths
consistent with the assumption of Keplerian rotation. Here we report a spectrum
from the Phoenix instrument on Gemini South which shows that differential
(slower) rotation continues to be observed out to ~ 5 micron. The observed
spectrum is well matched by the prediction of our accretion disk model
previously constructed to match the observed spectral energy distribution and
the differential rotation at wavelengths < 2.2 micron. This kinematic result
allows us to confirm our previous inference of a large outer radius (~ 1 AU)
for the rapidly accreting region of the FU Ori disk, which presents
difficulties for outburst models relying purely on thermal instability. While
some optical spectra have been interpreted to pose problems for the disk
interpretation of FU Ori, we show that the adjustment of the maximum effective
temperature of the disk model, proposed in a previous paper, greatly reduces
these difficulties.
