The Far-ultraviolet Spectrum of FU Ori South

Abstract The eruptive young stellar object FU Ori is the eponym of its variable class. FU Ori stars are known to undergo outbursts with amplitudes of >4 mag in the V band and durations of several decades. Interaction with a binary companion is one proposed outburst trigger, so understanding both components of the FU Ori system is crucial. A recent Hubble Space Telescope/Space Telescope Imaging Spectrograph observation of the FU Ori system clearly resolves its North and South components. We report here on the spectrum of FU Ori South. We detect near-ultraviolet continuum emission but no far-ultraviolet continuum, although several bright emission lines consistent with those seen in T Tauri stars are present. The presence of the C ii ] 2325 multiplet and many H 2 lines indicate active accretion. We estimate the extinction to the source and find that the UV spectrum favors A V < 4, contrary to past estimates based on the near-infrared spectrum.