The Astrophysical Journal Letters (Jan 2024)

A Far-ultraviolet-detected Accretion Shock at the Star–Disk Boundary of FU Ori

  • Adolfo S. Carvalho,
  • Lynne A. Hillenbrand,
  • Kevin France,
  • Gregory J. Herczeg

DOI
https://doi.org/10.3847/2041-8213/ad74eb
Journal volume & issue
Vol. 973, no. 2
p. L40

Abstract

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FU Ori objects are the most extreme eruptive young stars known. Their 4–5 mag 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 λ = 2300 Å. We present a new near-ultraviolet and the first high-sensitivity far-ultraviolet (FUV) spectrum of FU Ori. The FUV continuum is detected for the first time and, at λ = 1400 Å, 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 _FUV ≈ 16,000 ± 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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