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We report on recent Ks-band interferometric observations of the youngpre-main-sequence star AB Aurigae obtained with the Palomar Fiber Nuller (PFN).Reaching a contrast of a few 10-4 inside a field of view extending from 35 to275 mas (5-40AU at AB Aur's distance), the PFN is able to explore angularscales that are intermediate between those accessed by coronagraphic imagingand long baseline interferometry. This intermediate region is of specialinterest given that many young stellar objects are believed to harbor extendedhalos at such angular scales. Using destructive interference (nulling) betweentwo sub-apertures of the Palomar 200 inch telescope and rotating the telescopepupil, we measured a resolved circumstellar excess at all probed azimuthangles. The astrophysical null measured over the full rotation is fairlyconstant, with a mean value of 1.52%, and a slight additional azimuthalmodulation of +/-0.2%. The isotropic astrophysical null is indicative ofcircumstellar emission dominated by an azimuthally extended source, possibly ahalo, or one or more rings of dust, accounting for several percent of the totalKs-band flux. The modest azimuthal variation may be explained by some skewnessor anisotropy of the spatially extended source, e.g., an elliptical or spiralgeometry, or clumping, but it could also be due to the presence of a pointsource located at a separation of ~120 mas (17AU) with ~6*10-3 of the stellarflux. We combine our results with previous Infrared Optical Telescope Arrayobservations of AB Aur at H band, and demonstrate that a dust ring located at~30 mas (4.3AU) represents the best-fitting model to explain both sets ofvisibilities.We are also able to test a few previously hypothesized models ofthe incoherent component evident at longer interferometric baselines.

Exploring intermediate (5-40AU) scales around AB Aurigae with the Palomar Fiber Nuller