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

摘要

We present the first results from the polarimetry mode of the Gemini PlanetImager (GPI), which uses a new integral field polarimetry architecture toprovide high contrast linear polarimetry with minimal systematic biases betweenthe orthogonal polarizations. We describe the design, data reduction methods,and performance of polarimetry with GPI. Point spread function subtraction viadifferential polarimetry suppresses unpolarized starlight by a factor of over100, and provides sensitivity to circumstellar dust reaching the photon noiselimit for these observations. In the case of the circumstellar disk around HR4796A, GPI's advanced adaptive optics system reveals the disk clearly evenprior to PSF subtraction. In polarized light, the disk is seen all the way into its semi-minor axis for the first time. The disk exhibits surprisinglystrong asymmetry in polarized intensity, with the west side >9 times brighterthan the east side despite the fact that the east side is slightly brighter intotal 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 priorinterpretations. Forward scattering by relatively large silicate dust particlesleads to the strong polarized intensity on the west side, and the ring must beslightly optically thick in order to explain the lower brightness in totalintensity there. These findings suggest that the ring is geometrically narrowand dynamically cold, perhaps shepherded by larger bodies in the same manner asSaturn's F ring.