The case of AB Aurigae's disk in polarized light: Is there truly a gap?

Using the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) coronagraph, we have obtained high-contrast 2.0 μm imaging polarimetry and 1.1 μm imaging of the circumstellar disk around AB Aurigae on angular scales of 03-3″ (40-550 AU). Unlike previous observations, these data resolve the disk in both total and polarized intensity, allowing accurate measurement of the spatial variation of polarization fraction across the disk. Using these observations, we investigate the apparent "gap" in the disk reported by Oppenheimer et al.. In polarized intensity, the NICMOS data closely reproduce the morphology seen by Oppenheimer et al., yet in total intensity we find no evidence for a gap in either our 1.1 or 2.0 μm images. We find instead that region has lower polarization fraction, without a significant decrease in total scattered light, consistent with expectations for backscattered light on the far side of an inclined disk. Radiative transfer models demonstrate this explanation fits the observations. Geometrical scattering effects are entirely sufficient to explain the observed morphology without any need to invoke a gap or a protoplanet at that location.