HR 4796 A's Circumstellar Disk Reexamined

New constraints on the dust surrounding HR 4796 A

Authors:

Milli et al

Abstract:

HR 4796 A is surrounded by a well-structured and very bright circumstellar disc shaped as an annulus with many interesting features: very sharp inner and outer edges, brightness asymmetries, center offset and suspected distortions in the ring. We aim to constrain the properties of the dust surrounding the star HR 4796A, in particular the grain size and composition. We also want to confirm and refine the morphological parameters derived from previous scattered light observations, and reveal the dust spatial extent in regions unexplored so far due to their proximity to the star. We have obtained new images in polarized light of the binary system HR\ 4796A and B in the Ks and Lp band with the NaCo instrument at the Very Large Telescope (VLT). In addition, we revisit two archival data obtained in the Lp band with that same instrument and at 2.2 microns with the NICMOS instrument on the Hubble Space Telescope. We complement these observations with simulations using the radiative transfer code MCFOST to investigate the dust properties. We explore a grid of models with various dust composition and sizes. We detect the disc in polarized light in the Ks band and reveal for the first time the innermost regions down to 0.3" along the minor axis. We measure a polarized fraction of 29%±8% in the two disc ansae, with a maximum occurring more than 13∘ westwards from the ansae. A very pronounced brightness asymmetry between the North-West and South-East side is detected. This contradicts the asymmetry previously reported in all images of the disc in unpolarized light at wavelengths smaller than or equal to 2.2 microns and is inconsistent with the predicted scattered light from spherical grains using the Mie theory.