  We report on the detection of localized off-center emission at 1-4 AU in the
circumstellar environment of the young stellar object AB Aurigae. We used
closure phase measurements in the near-infrared made at the long baseline
interferometer IOTA, the first obtained on a young stellar object using this
technique. When probing sub-AU scales, all closure phases are close to zero
degrees, as expected given the previously-determined size of the AB Aurigae
inner dust disk. However, a clear closure phase signal of -3.5 +/- 0.5 degrees
is detected on one triangle containing relatively short baselines, requiring a
high degree of non-point symmetry from emission at larger (AU-sized) scales in
the disk. We have not identified any alternative explanation for these closure
phase results and demonstrate that a ``disk hot spot'' model can fit our data.
We speculate that such asymmetric near-infrared emission detected might arise
as a result of localized viscous heating due to a gravitational instability in
the AB Aurigae disk, or to the presence of a close stellar companion or
accreting sub-stellar object.

Astrophysics