The Near-Ultraviolet Spectra of FU Orionis Accretion Disks

TL;DR

This study presents the first high-sensitivity near-ultraviolet spectra of FU Orionis objects, revealing excess emission inconsistent with standard disk models and providing insights into accretion processes and boundary layer absence.

Contribution

It offers new high-resolution UV spectra of FU Ori objects, demonstrating excess emission and ruling out hot boundary layers, advancing understanding of accretion mechanisms in these stars.

Findings

NUV excess emission is consistent across sources.

Hot boundary layer is conclusively ruled out.

Detected unique UV emission lines in FU Ori objects.

Abstract

We present the results of the first high-sensitivity NUV (1800 to 3200 \AA) survey of FU Ori objects, using the \textit{Hubble Space Telescope} (HST) STIS spectrograph. We compare new low resolution spectra for 6 sources with predictions from accretion disk models and find that all show emission in excess of the disk model spectrum. The physical properties of the NUV emission excess are very consistent among the sample, with a mean luminosity of $10^{-1.11 \pm 0.4} \ L_\odot$ and temperature of $16400 \pm 2600$ K -- despite spanning 0.9 dex in $M_*$, 1.3 dex in $\dot{M}$, and 0.7 dex in $L_\mathrm{acc}$. We use the spectra to conclusively rule out the existence of a hot boundary layer in FU Ori accretion disks. We then discuss the source of the excess emission in the context of recent simulations of FU Ori outbursts and boundary layer accretion. The UV spectra also show the often-seen \ion{C}{2}] 2326 \AA\ multiplet and \ion{Mg}{2} 2796/2803 \AA\ doublet, as well as the unusual \ion{Fe}{2}] 2507/2509 \AA\ doublet, a feature that is not seen in the existing UV spectra of other young stellar objects. We measure and compare the luminosities of these lines in outbursting with those in non-outbursting objects.