Speckle suppression through dual imaging polarimetry, and a ground-based image of the hr 4796a circumstellar disk

Abstract

We demonstrate the versatility of a dual imaging polarimeter working in tandem with a Lyot coronagraph and adaptive optics to suppress the highly static speckle noise pattern - the greatest hindrance to ground-based direct imaging of planets and disks around nearby stars. Using a double difference technique with the polarimetric data, we quantify the level of speckle suppression, and hence improved sensitivity, by placing an ensemble of artificial faint companions into real data, with given total brightness and polarization. For highly polarized sources within 05, we show that we achieve 3 to 4mag greater sensitivity through polarimetric speckle suppression than simply using a coronagraph coupled to a high-order adaptive optics system. Using such a polarimeter with a classical Lyot coronagraph at the 3.63 m Advanced Electro-Optical System telescope, we have obtained a 6.5σ detection in the Hband of the 76 AU diameter circumstellar debris disk around the star HR 4796A. Our data represent the first definitive ground-based near-IR polarimetric image of the HR 4796A debris disk and clearly show the two outer ansae of the disk, evident in Hubble Space Telescope NICMOS/STIS imaging. Comparing our peak linearly polarized flux with the total intensity in the lobes as observed by NICMOS, we derive a lower limit to the fractional linear polarization of >29% caused by dust grains in the disk. In addition, we fit simple morphological models of optically thin disks to our data allowing us to constrain the dust disk scale height (2.5+5.0-1.3 AU) and scattering asymmetry parameter (g = 〈cos θ〉 = 0.20+.07-.10). These values are consistent with several lines of evidence suggesting that the HR 4796A disk is dominated by a micron-sized dust population, and are indeed typical of disks in transition between those surrounding the Herbig Ae stars to those associated with Vega-like stars. © 2009 The American Astronomical Society. All rights reserved.