The Case of AB Aurigae's Disk in Polarized Light: Is There Truly a Gap?

TL;DR

This study uses high-contrast polarimetric imaging to analyze the AB Aurigae disk, revealing that the previously reported gap is likely a result of scattering effects rather than an actual disk gap or protoplanet.

Contribution

The paper demonstrates that the apparent disk gap can be explained by scattering effects, challenging previous interpretations of the disk structure.

Findings

No evidence of a gap in total intensity images.

Lower polarization fraction explains the apparent gap.

Radiative transfer models support scattering effects as the cause.

Abstract

Using the NICMOS coronagraph, we have obtained high-contrast 2.0 micron imaging polarimetry and 1.1 micron imaging of the circumstellar disk around AB Aurigae on angular scales of 0.3-3 arcsec (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. 2008. In polarized intensity, the NICMOS data closely reproduces 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 micron images. We find instead that region has lower polarization fraction, without a significant decrease in total scattered light, consistent with expectations for back-scattered 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 protoplanet at that location.