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New constraints on the dust surrounding HR 4796A

Milli, J. and Mawet, D. and Lagrange, A.-M. and Mouillet, D. and Girard, J. H. and Augereau, J.-C. and De Boer, J. and Pueyo, L. and Choquet, É. (2015) New constraints on the dust surrounding HR 4796A. Astronomy and Astrophysics, 577 . A57. ISSN 0004-6361. doi:10.1051/0004-6361/201423950.

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Context. 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. Aims. 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. Methods. We have obtained new images in polarised light of the binary system HR 4796A and B in the Ks and L′ band with the NaCo instrument at the Very Large Telescope (VLT). In addition, we revisit two archival data sets obtained in the L′ band with that same instrument and at 2.2 μm 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. Results. 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% ± 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 unpolarised light at wavelengths smaller than or equal to 2.2 μm 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. It shows that the dust is composed of porous aggregates larger than 1 μm.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Milli, J.0000-0001-9325-2511
Mawet, D.0000-0002-8895-4735
Girard, J. H.0000-0001-8627-0404
Pueyo, L.0000-0003-3818-408X
Choquet, É.0000-0002-9173-0740
Additional Information:© 2015 ESO. Received 7 April 2014. Accepted 9 February 2015. J.M. acknowledges financial support from the ESO studentship program and the Labex OSUG2020. C. Pinte acknowledges funding from the Agence Nationale pour la Recherche (ANR) of France under contract ANR-2010-JCJC-0504-01. We would like to thank John Krist for his initial help with the NICMOS data and we thank ESO staff and technical operators at the Paranal Observatory.
Funding AgencyGrant Number
European Southern Observatory (ESO)UNSPECIFIED
Agence Nationale de la Recherche (ANR)ANR-2010-JCJC-0504-01
Subject Keywords:instrumentation: high angular resolution; planetary systems; stars: individual: HR 4796A; scattering; instrumentation: polarimeters
Record Number:CaltechAUTHORS:20150602-095151702
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Official Citation:J. Milli, D. Mawet, C. Pinte, A.-M. Lagrange, D. Mouillet, J. H. Girard, J.-C. Augereau, J. De Boer, L. Pueyo and É. Choquet A&A 577 A57 (2015) DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57932
Deposited On:02 Jun 2015 17:37
Last Modified:10 Nov 2021 21:56

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