Very deep images of the innermost regions of the β Pictoris debris disc at L

Context. Very few debris discs have been imaged in scattered light at wavelengths beyond 3 μm because the thermal emission from both the sky and the telescope is generally too strong with respect to the faint emission of a debris disc. We present here the first analysis of a high angular resolution image of the disc of β Pictoris at 3.8 μm. Aims. Our primary objective is to probe the innermost parts of the β Pictoris debris disc and describe its morphology. We performed extensive forward modelling to correct for the biases induced by angular differential imaging on extended objects and derive the physical parameters of the disc. Methods. This work relies on a new analysis of seven archival data sets of β Pictoris observed with the NaCo instrument at the Very Large Telescope in the L′ band, including observations made with the Annular Groove Phase Mask vortex coronagraph in 2013. The data analysis consists of angular differential imaging associated with disc forward modelling to correct for the biases induced by that technique. The disc model is subtracted from the data and the reduction performed again in order to minimize the residuals in the final image. Results. The disc is detected above a 5σ level between 0.4′′ and 3.8′′. The two extensions have a similar brightness within error bars. We confirm an asymmetry previously observed at larger distances from the star and at shorter wavelengths: the isophotes are more widely spaced on the north-west side (above the disc apparent midplane) than on the south-east side. This is interpreted as a small inclination of the disc combined with anisotropic scattering. Our best-fit model has an inclination of 86° with an anisotropic Henyey-Greenstein coefficient of 0.36. This interpretation is supported by a new asymmetry detected in the disc: the disc is significantly bowed towards the north-west within 3″ (above the apparent midplane). We also detect a possible new asymmetry within 1″, but at this stage we cannot discern between a real feature and an underlying speckle.

[1]  Bradford A. Smith,et al.  A Circumstellar Disk Around β Pictoris , 1984, Science.

[2]  H. H. Aumann,et al.  IRAS OBSERVATIONS OF MATTER AROUND NEARBY STARS. , 1985 .

[3]  Paul Kalas,et al.  Asymmetries in the Beta Pictoris Dust Disk , 1995 .

[4]  D. Mouillet,et al.  A planet on an inclined orbit as an explanation of the warp in the β Pictoris disc , 1997 .

[5]  R. P. Nelson,et al.  Dynamical modeling of large scale asymmetries in the beta Pictoris dust disk , 2001 .

[6]  The Inner Rings of β Pictoris , 2002, astro-ph/0212081.

[7]  B. Draine.  Scattering by Interstellar Dust Grains. I. Optical and Ultraviolet , 2003, astro-ph/0304060.

[8]  Shigeyuki Sako,et al.  An early extrasolar planetary system revealed by planetesimal belts in β Pictoris , 2004, Nature.

[9]  Pierre Kervella,et al.  VLTI near-IR interferometric observations of Vega-like stars: Radius and age of a PsA, b Leo, b Pic, e Eri and t Cet , 2004 .

[10]  Thomas L. Hayward,et al.  Mid-infrared images of beta Pictoris and the possible role of planetesimal collisions in the central disk. , 2005, Nature.

[11]  B. Macintosh,et al.  Angular Differential Imaging: A Powerful High-Contrast Imaging Technique , 2005, astro-ph/0512335.

[12]  Mark Clampin,et al.  Discovery of an 86 AU Radius Debris Ring around HD 181327 , 2006 .

[13]  M. Franx,et al.  Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris , 2006 .

[14]  Etienne Artigau,et al.  A New Algorithm for Point-Spread Function Subtraction in High-Contrast Imaging: A Demonstration with Angular Differential Imaging , 2007, astro-ph/0702697.

[15]  P. Thebault,et al.  Outer edges of debris discs - How sharp is sharp? , 2008, 0801.3724.

[16]  B. Croll,et al.  DUST DISTRIBUTION IN THE β PICTORIS CIRCUMSTELLAR DISKS , 2009, 0909.0730.

[17]  A. Boccaletti,et al.  VLT/NACO coronagraphic observations of fine structures in the disk of β Pictoris , 2009, 0901.2034.

[18]  A. Boccaletti,et al.  A Giant Planet Imaged in the Disk of the Young Star β Pictoris , 2010, Science.

[19]  G. Montagnier,et al.  Dissecting the Moth: discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging , 2010, 1011.2496.

[20]  Subaru Telescope,et al.  IMAGES OF THE EXTENDED OUTER REGIONS OF THE DEBRIS RING AROUND HR 4796 A , 2011, 1110.2488.

[21]  Julien H. Girard,et al.  High angular resolution detection of β Pictoris b at 2.18 μm , 2011 .

[22]  Armando Riccardi,et al.  THE GRAY NEEDLE: LARGE GRAINS IN THE HD 15115 DEBRIS DISK FROM LBT/PISCES/Ks AND LBTI/LMIRcam/L′ ADAPTIVE OPTICS IMAGING , 2012, 1203.2619.

[23]  J. Augereau,et al.  An icy Kuiper-Belt around the young solar-type star HD 181327 , 2011, 1112.3398.

[24]  D. Mawet,et al.  Impact of angular differential imaging on circumstellar disk images , 2012, 1207.5909.

[25]  David Mouillet,et al.  Morphology of the very inclined debris disk around HD 32297 , 2012, 1206.6334.

[26]  A. Boccaletti,et al.  The position of β Pictoris b position relative to the debris disk , 2012, 1202.2578.

[27]  A. Boccaletti,et al.  An insight in the surroundings of HR 4796 , 2012, 1207.1987.

[28]  Adam Amara,et al.  PYNPOINT: An image processing package for finding exoplanets , 2012, 1207.6637.

[29]  R. Soummer,et al.  DETECTION AND CHARACTERIZATION OF EXOPLANETS AND DISKS USING PROJECTIONS ON KARHUNEN–LOÈVE EIGENIMAGES , 2012, 1207.4197.

[30]  C. Dominik,et al.  Why circumstellar disks are so faint in scattered light: the case of HD 100546 , 2012, 1210.4132.

[31]  D. Mawet,et al.  Searching for companions down to 2 AU from β Pictoris using the L′-band AGPM coronagraph on VLT/NACO , 2013, 1311.4298.

[32]  James R. Graham,et al.  MODELING SELF-SUBTRACTION IN ANGULAR DIFFERENTIAL IMAGING: APPLICATION TO THE HD 32297 DEBRIS DISK , 2013, 1310.7026.

[33]  Julien H. Girard,et al.  A COMBINED VERY LARGE TELESCOPE AND GEMINI STUDY OF THE ATMOSPHERE OF THE DIRECTLY IMAGED PLANET, β PICTORIS b , 2013, 1306.0610.

[34]  Julien H. Girard,et al.  L'-band AGPM vector vortex coronagraph's first light on VLT/NACO. Discovery of a late-type companion , 2013, 1304.0930.

[35]  Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk , 2014, Science.

[36]  National Astronomical Observatory of Japan,et al.  The architecture of the LkCa 15 transitional disk revealed by high-contrast imaging⋆ , 2014, 1402.1766.