  We report on recent Ks-band interferometric observations of the young
pre-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 to
275 mas (5-40AU at AB Aur's distance), the PFN is able to explore angular
scales that are intermediate between those accessed by coronagraphic imaging
and long baseline interferometry. This intermediate region is of special
interest given that many young stellar objects are believed to harbor extended
halos at such angular scales. Using destructive interference (nulling) between
two sub-apertures of the Palomar 200 inch telescope and rotating the telescope
pupil, we measured a resolved circumstellar excess at all probed azimuth
angles. The astrophysical null measured over the full rotation is fairly
constant, with a mean value of 1.52%, and a slight additional azimuthal
modulation of +/-0.2%. The isotropic astrophysical null is indicative of
circumstellar emission dominated by an azimuthally extended source, possibly a
halo, or one or more rings of dust, accounting for several percent of the total
Ks-band flux. The modest azimuthal variation may be explained by some skewness
or anisotropy of the spatially extended source, e.g., an elliptical or spiral
geometry, or clumping, but it could also be due to the presence of a point
source located at a separation of ~120 mas (17AU) with ~6*10-3 of the stellar
flux. We combine our results with previous Infrared Optical Telescope Array
observations 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 of
visibilities.We are also able to test a few previously hypothesized models of
the incoherent component evident at longer interferometric baselines.
