An FUV-detected Accretion Shock at the Star-Disk Boundary of FU Ori

TL;DR

This study presents the first high-sensitivity FUV spectrum of FU Ori, revealing a shock at the star-disk boundary with a temperature of about 16,000 K, providing new insights into accretion processes in eruptive young stars.

Contribution

It provides the first detection of FUV continuum in FU Ori and models the boundary shock emission, advancing understanding of accretion in FU Ori objects.

Findings

FUV continuum detected at 1400 Å, over 10,000 times brighter than disk models

Shock temperature estimated at approximately 16,000 K

Inferred accretion flow velocity at the boundary is about 40 km/s

Abstract

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.