Space-Based Coronagraphic Imaging Polarimetry of the TW Hydrae Disk: Shedding New Light on Self-Shadowing Effects

TL;DR

This study uses Hubble Space Telescope polarimetry to analyze the TW Hydrae disk, revealing azimuthal asymmetries and variations in polarization fraction consistent with self-shadowing by an inclined inner disk, advancing understanding of disk structures.

Contribution

It provides the first detailed polarimetric imaging evidence supporting self-shadowing effects caused by an inclined inner disk in the TW Hydrae system.

Findings

Azimuthal asymmetries in total and polarized intensity consistent with bright and dark rings.

Significant radial and azimuthal variations in polarization fraction linked to disk structures.

Azimuthal polarization variation attributed to decreased multiple scattering due to shadowing.

Abstract

We present Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer coronagraphic imaging polarimetry of the TW Hydrae protoplanetary disk. These observations simultaneously measure the total and polarized intensity, allowing direct measurement of the polarization fraction across the disk. In accord with the self-shadowing hypothesis recently proposed by Debes et al., we find that the total and polarized intensity of the disk exhibits strong azimuthal asymmetries at projected distances consistent with the previously reported bright and dark ring-shaped structures (~45-99 au). The sinusoidal-like variations possess a maximum brightness at position angles near ~268-300 degrees and are up to ~28% stronger in total intensity. Furthermore, significant radial and azimuthal variations are also detected in the polarization fraction of the disk. In particular, we find that regions of lower polarization fraction are associated with annuli of increased surface brightness, suggesting that the relative proportion of multiple-to-single scattering is greater along the ring and gap structures. Moreover, we find strong (~20%) azimuthal variation in the polarization fraction along the shadowed region of the disk. Further investigation reveals that the azimuthal variation is not the result of disk flaring effects, but instead from a decrease in the relative contribution of multiple-to-single scattering within the shadowed region. Employing a two-layer scattering surface, we hypothesize that the diminished contribution in multiple scattering may result from shadowing by an inclined inner disk, which prevents direct stellar light from reaching the optically thick underlying surface component.