The Near-Ultraviolet Spectra of FU Orionis Accretion Disks

TL;DR

This study tests the existence of a boundary-layer UV excess in FU Ori accretion disks by carrying out a high-sensitivity NUV spectroscopic survey with HST/STIS across six FU Ori objects. The spectra show a UV emission excess beyond the viscous disk, with a mean excess luminosity of $L oughly 10^{-1.11 \pm 0.4} Lsun$ and a characteristic temperature of $T \approx 16400 \pm 2600$ K, challenging purely hot boundary-layer models. The authors discuss possible origins of the excess in the context of boundary-layer and magnetic heating scenarios and report UV emission lines such as C II] 2326 Å, Mg II 2796/2803 Å, and Fe II] 2507/2509 Å, comparing line luminosities to non-outbursting objects. The UV excess correlates with system luminosity, linking the excess to the accretion outburst itself and offering diagnostics for accretion physics in FU Ori systems, thereby constraining boundary-layer models for high-$\dot M$ young stellar objects.

Abstract

We present the results of the first high-sensitivity NUV (1800 to 3200 Å) 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 Å multiplet and \ion{Mg}{2} 2796/2803 Å doublet, as well as the unusual \ion{Fe}{2}] 2507/2509 Å 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.