New insights on the AU-scale circumstellar structure of FU Orionis

We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 m in length, in the H and K bands. Our data resolves FU Ori at the AU scale, and provides new constraints at shorter baselines and shorter wavelengths. Our extensive (u, v)- plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models and with an accretion rate of. M = (6.3 +/- 0.6) x 10(-5) (M-* M-circle dot)(-1) M-circle dot yr(-1). We are able to constrain the geometry of the disk, including an inclination of 55(-7)(+5) deg and a position angle of 47(-11)(+7) deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data ( at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. The still somewhat limited ( u, v) sampling and substantial measurement uncertainty prevent us from constraining the location of the spot with confidence, since many solutions yield a statistically acceptable fit. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 +/- 1 AU at the 130 +/- 1 deg position angle and with a magnitude difference of Delta K approximate to 3.9 +/- 0.2 and Delta H approximate to 3.6 +/- 0.2 mag moving away from the center at a rate of 1.2 +/- 0.6 AU yr(-1). Although this bright spot on the surface of the disk could be tracing some thermal instabilities in the disk, we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object.