FU Orionis resolved by infrared long-baseline interferometry at a 2 AU scale

We present the first infrared interferometric observations of a young stellar object with a spatial projected resolution better than 2 AU. The observations were obtained with the Palomar Testbed Interferometer (PTI). FU Orionis exhibits a visibility of V-2 = 0.72 +/- 0.07 for a 103 +/- 5 m-projected baseline at lambda = 2.2 mu m. On the spatial scale probed by the PTI, the data are consistent with both a binary system scenario (a maximum magnitude difference of 2.7 +/- 0.5 mag and the smallest separation of 0.35 +/- 0.05 AU) and a standard luminous accretion disk model ((M)over dot similar to 6 x 10(-5) M. yr(-1)), where the thermal emission dominates the stellar scattering, and are inconsistent with a single stellar photosphere.