New constraints on the dust surrounding HR 4796A

Abstract

HR 4796A is surrounded by a well-structured and very bright circumstellar disc shaped like an annulus with many interesting features: very sharp inner and outer edges, brightness asymmetries, centre 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 polarised 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 sets obtained in the Lp band with that same instrument and at 2.2 microns with the NICMOS instrument on the Hubble Space Telescope. We analyse these observations with simulations using the radiative transfer code MCFOST to investigate the dust properties. We explore a grid of models with various dust compositions and sizes in a Bayesian approach. We detect the disc in polarised light in the Ks band and reveal for the first time the innermost regions down to $0.3"$ along the semi-minor axis. We measure a polarised fraction of $29\%\pm 8\%$ in the two disc ansae, with a maximum occurring more than $13^{\circ }$ 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 unpolarised 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. Our modelling suggests the north-west side is most likely inclined towards the Earth, contrary to previous conclusions.

17 pages, 12 figures

References in corpus (11)

• The Gemini Planet Imager: First Light
• Monte Carlo radiative transfer in protoplanetary disks
• Molecular Gas Clumps from the Destruction of Icy Bodies in the $\beta$ Pictoris Debris Disk
• Polarimetry with the Gemini Planet Imager: Methods, Performance at First Light, and the Circumstellar Ring around HR 4796A
• Effective medium theories for irregular fluffy structures: aggregation of small particles
• STIS Imaging of the HR 4796A Circumstellar Debris Ring
• Speckle Suppression Through Dual Imaging Polarimetry, and a Ground-Based Image of the HR 4796A Circumstellar Disk
• Complex Organic Materials in the Circumstellar Disk of HR 4796A
• Five Debris Disks Newly Revealed in Scattered Light from the HST NICMOS Archive
• Very deep images of the innermost regions of the beta Pictoris debris disc at Lp
• The Gemini NICI Planet-Finding Campaign: The Offset Ring of HR 4796 A

Cited by in corpus (15)

• Near-infrared scattered light properties of the HR4796A dust ring: a measured scattering phase function from 13.6deg to 166.6deg
• The polarimetric imaging mode of VLT/SPHERE/IRDIS II: Characterization and correction of instrumental polarization effects
• Exocometary gas structure, origin and physical properties around $\beta$ Pictoris through ALMA CO multi-transition observations
• The polarimetric imaging mode of VLT/SPHERE/IRDIS I: Description, data reduction and observing strategy
• Optical polarized phase function of the HR\,4796A dust ring
• The SHARDDS survey: first resolved image of the HD114082 debris disk in Lower Centaurus Crux with SPHERE
• Characterization of the inner disk around HD 141569 A from Keck/NIRC2 L-band vortex coronagraphy
• HST Scattered Light Imaging and Modeling of the Edge-on Protoplanetary Disk ESO-H$\alpha$ 569
• Debris Disk Color with the Hubble Space Telescope
• The influence of dust grain porosity on the analysis of debris disc observations
• Adaptive optics in high-contrast imaging
• Polarimetric differential imaging with VLT/NACO. A comprehensive PDI pipeline for NACO data (PIPPIN)
• Do twin spectral peaks of olivine particles in the thermal infrared diagnose their sizes and porosities?
• The Disks In Scorpius-Centaurus Survey (DISCS) I: Four Newly-Resolved Debris Disks in Polarized Intensity Light
• A Multiband Study of the HR 4796A Disk in the Optical Using MagAO-X