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 0.5 ^ { \prime \prime } , we show that we achieve 3 to 4 magnitudes 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 AEOS telescope , we have obtained a 6.5 \sigma detection in the H -band 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 \theta > = 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 .