Polarimetry with the Gemini Planet Imager: Methods, Performance at First Light, and the Circumstellar Ring around HR 4796A

TL;DR

This paper introduces the first polarimetry results from the Gemini Planet Imager, demonstrating high contrast imaging of circumstellar disks, revealing new asymmetries and insights into the structure and composition of the HR 4796A ring.

Contribution

It presents a novel integral field polarimetry architecture for GPI, achieving high contrast polarimetry with minimal biases and revealing detailed disk structures.

Findings

The disk around HR 4796A is visible all the way to its semi-minor axis in polarized light.

The west side of the disk is over 9 times brighter in polarized intensity than the east side.

The ring is likely geometrically narrow, optically thick, and possibly shepherded by larger bodies.

Abstract

We present the first results from the polarimetry mode of the Gemini Planet Imager (GPI), which uses a new integral field polarimetry architecture to provide high contrast linear polarimetry with minimal systematic biases between the orthogonal polarizations. We describe the design, data reduction methods, and performance of polarimetry with GPI. Point spread function subtraction via differential polarimetry suppresses unpolarized starlight by a factor of over 100, and provides sensitivity to circumstellar dust reaching the photon noise limit for these observations. In the case of the circumstellar disk around HR 4796A, GPI's advanced adaptive optics system reveals the disk clearly even prior to PSF subtraction. In polarized light, the disk is seen all the way in to its semi-minor axis for the first time. The disk exhibits surprisingly strong asymmetry in polarized intensity, with the west side >9 times brighter than the east side despite the fact that the east side is slightly brighter in total intensity. Based on a synthesis of the total and polarized intensities, we now believe that the west side is closer to us, contrary to most prior interpretations. Forward scattering by relatively large silicate dust particles leads to the strong polarized intensity on the west side, and the ring must be slightly optically thick in order to explain the lower brightness in total intensity there. These findings suggest that the ring is geometrically narrow and dynamically cold, perhaps shepherded by larger bodies in the same manner as Saturn's F ring.