Exploring 5-40 AU scales around AB Aurigae with an upgraded Palomar Fiber Nuller

With a null precision of a few 10-4 at all azimuth angles inside a field-of-view extending from 35 to 275 mas, the Palomar Fiber Nuller (PFN) is able to explore angular scales intermediate between those accessed by coronagraphic imaging and by long baseline interferometry. We first briefly summarize the recent performance improvements of the PFN (sensitivity, azimuthal coverage, duty cycle efficiency on-sky) over the 2011-2014 time period. Then we report on recent K-band observations of the young pre-main sequence star AB Aurigae obtained with the PFN. It is shown that a mean astrophysical null of 1.52% was detected around AB Aur at all probed azimuthal angles, and this inside a field-of-view corresponding to projected separations between 5 and 40 AU. In addition, we also report a slight +/- 0.2% modulation in addition to this average null level. 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. The modest azimuthal variation may be explained by some skewness or anisotropy of the spatially-extended source, e.g. with 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 similar to 120 mas (17AU) and carrying similar to 6*10-3 of the stellar flux.