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We report on the detection of localized off-center emission at 1-4 AU in thecircumstellar environment of the young stellar object AB Aurigae. We usedclosure phase measurements in the near-infrared made at the long baselineinterferometer IOTA, the first obtained on a young stellar object using thistechnique. When probing sub-AU scales, all closure phases are close to zerodegrees, as expected given the previously-determined size of the AB Aurigaeinner dust disk. However, a clear closure phase signal of -3.5 +/- 0.5 degreesis detected on one triangle containing relatively short baselines, requiring ahigh degree of non-point symmetry from emission at larger (AU-sized) scales inthe disk. We have not identified any alternative explanation for these closurephase results and demonstrate that a ``disk hot spot'' model can fit our data.We speculate that such asymmetric near-infrared emission detected might ariseas a result of localized viscous heating due to a gravitational instability inthe AB Aurigae disk, or to the presence of a close stellar companion oraccreting sub-stellar object.Comment: Accepted by Astrophysical Journal Letter

Bright Localized Near-Infrared Emission at 1-4 AU in the AB Aurigae Disk Revealed by IOTA Closure Phases