instrumentation: high angular resolution; planet-disk interactions; planets and satellites: detection; scattering; planetary systems; Astrophysics - Earth and Planetary Astrophysics
Abstract :
[en] Context. HR 4796 A is surrounded by a debris disc, observed in scattered light as an inclined ring with a high surface brightness. Past observations have raised several questions. First, a strong brightness asymmetry detected in polarised reflected light has recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date. <BR /> Aims: We aim here at measuring with high accuracy the morphology and photometry of the ring in scattered light, in order to derive the phase function of the dust and constrain its near-infrared spectral properties. We also want to constrain the presence of planets and set improved constraints on the origin of the observed ring morphology. <BR /> Methods: We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR 4796 A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise. <BR /> Results: We determine empirically the scattering phase function of the dust in the H band from 13.6° to 166.6°. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30°. We analyse the reflectance spectra of the disc from the 0.95 μm to 1.6 μm, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects. <BR /> Conclusions: We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function, especially below 45°, suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 μm. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50° points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star, but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Milli, J.; European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile ; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Vigan, A.; Aix-Marseille Univ, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Mouillet, D.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Lagrange, A.-M.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Augereau, J.-C.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Pinte, C.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France ; UMI-FCA, CNRS/INSU France (UMI 3386), and Departamento de Astronomia, Universidad de Chile, Casilla 36-, D Santiago, Chile
Mawet, D.; Department of Astronomy, California Institute of Technology, 1200 E. California Blvd, MC 249-17, Pasadena, CA, 91125, USA ; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
Schmid, H. M.; ETH Zurich, Institute for Astronomy, 8093, Zurich, Switzerland
Boccaletti, A.; LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92190, Meudon, France
Matrà, L.; Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
Kral, Q.; Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
Ertel, S.; Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ, 85719, USA
Chauvin, G.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Bazzon, A.; ETH Zurich, Institute for Astronomy, 8093, Zurich, Switzerland
Ménard, F.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France ; UMI-FCA, CNRS/INSU France (UMI 3386), and Departamento de Astronomia, Universidad de Chile, Casilla 36-, D Santiago, Chile
Beuzit, J.-L.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
Thalmann, C.; ETH Zurich, Institute for Astronomy, 8093, Zurich, Switzerland
Dominik, C.; Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE, Amsterdam, The Netherlands
Feldt, M.; Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany
Henning, T.; Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany
Min, M.; Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE, Amsterdam, The Netherlands ; SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
Girard, J. H.; European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
Galicher, Raphaël; LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92190, Meudon, France
Bonnefoy, M.; Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
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