Kavli Affiliate: Gregory J. Herczeg
| First 5 Authors: Adolfo S. Carvalho, Lynne A. Hillenbrand, Kevin France, Gregory J. Herczeg,
| Summary:
FU Ori objects are the most extreme eruptive young stars known. Their 4 to 5
magnitude photometric outbursts last for decades and are attributed to a factor
of up to 10,000 increase in the stellar accretion rate. The nature of the
accretion disk-to-star interface in FU Ori objects has remained a mystery for
decades. To date, attempts to directly observe a shock or boundary layer have
been thwarted by the apparent lack of emission in excess of the accretion disk
photosphere down to $lambda = 2300$ AA. We present a new NUV and the first
high-sensitivity FUV spectrum of FU Ori. The FUV continuum is detected for the
first time and, at $lambda = 1400$ AA, is more than $10^4$ times brighter
than predicted by a viscous accretion disk. We interpret the excess as arising
from a shock at the boundary between the disk and the stellar surface. We model
the shock emission as a blackbody and find that the temperature of the shocked
material is $T_mathrm{FUV} approx 16,000 pm 2000$ K. The shock temperature
corresponds to an accretion flow along the surface of the disk that reaches a
velocity of 40 km s$^{-1}$ at the boundary, consistent with predictions from
simulations.
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