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The FU Orionis Binary System and the Formation of Close Binaries

The faint star next to FU Orionis recently discovered by Wang et al. has been observed with adaptive optics at the Subaru Telescope. Infrared JHK′L′ photometry shows clear infrared excess, indicating that the object is a pre-main-sequence star. Its infrared K-band spectrum is very different from that of FU Orionis and suggests a star of spectral type K. We discuss these observations in light of the hypothesis that FUors may be newborn binaries that have become bound when a small nonhierarchical multiple system breaks up. This scenario predicts that FU Orionis must be a close binary (<10 AU), and if so the newly discovered companion is the outlying member in a triple system. We discuss various implications of this scenario; in particular, we note that FUor eruptions should commonly occur during a relatively brief evolutionary phase partly overlapping with and immediately following the formation of Herbig-Haro jets. If this is the dominant mechanism to trigger FUor outbursts, then FUor eruptions should preferentially occur in close binaries, i.e., in about 20% of all stars.