2012AJ....143...55B

In this paper, we present the results of our adaptive optics fed three-dimensional imaging spectroscopy study of the FU Orionis binary system. Although the 0".5 separation companion to FU Ori is ∼4 mag fainter, we have easily spatially resolved it in the J, H, and K infrared bands and extract high signal-to-noise spectra of the two stellar components from 1.15 to 2.4 µm. We derive a spectral type of K5⁺²_- 1_ for FU Ori S based on the stellar photospheric absorption features and find that it is an actively accreting young star ({dot}M_acc ∼ (2-3)x10^–8 M_☉/yr) that is likely gravitationally bound to FU Ori. We have found that the continuum shape of FU Ori S is not well fit by our spectral modeling process, and this results in a large uncertainty in the line-of-sight extinction to the star. Yet, by placing FU Ori S on the Hertzsprung-Russell diagram and comparing its estimated location with evolutionary models, we find that it is best fit as a ∼1.2 M_☉star with a likely age of less than ∼2 Myr. If we assume coevality of the stellar components, we have thus placed an estimated age on the FU Ori system. Moreover, assuming the canonical model for the nature of FU Ori in that its optical and infrared absorption features arise primarily from the inner circumstellar disk around a ∼0.3 M_☉star, we find that the fainter FU Ori S component is actually the more massive star in the system. Future monitoring of FU Ori S to investigate flux variability and orbital motion should further clarify the nature of this curious young binary.