The Near-ultraviolet Spectra of FU Orionis Accretion Disks

We present the results of the first high-sensitivity near-ultraviolet (NUV; 1800–3200 Å) survey of FU Ori objects, using the Hubble Space Telescope STIS spectrograph. We compare new low-resolution spectra for six 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±0.4 L_⊙ and temperature of 16,400 ± 2600 K—despite spanning 0.9 dex in M_*, 1.3 dex in M, and 0.7 dex in L_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 C ii] 2326 Å multiplet and Mg ii 2796/2803 Å doublet, as well as the unusual Fe ii] 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 nonoutbursting objects.