An icy Kuiper belt around the young solar-type star HD 181327

¹ UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: jeremy.lebreton@obs.ujf-grenoble.fr
² Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD, 20771, USA
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁵ Centre for Astrophysics & Supercomputing, Swinburne University, PO, Box 218, Hawthorn, VIC 3122, Australia
⁶ LAEX, Depto. Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Spain
⁷ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁸ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁹ ESO-ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
¹⁰ Calar Alto Observatory, Centro Astronómico Hispano-Alemán, C/Jesús Durbán Remón, 2-2, 04004 Almería, Spain
¹¹ Astronomy Department, University of California, Berkeley CA 94720-3411, USA
¹² Université de Lyon, 69003 Lyon; Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint-Genis Laval ; CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon; École Normale Supérieure de Lyon, 69007 Lyon, France
¹³ Eureka Scientific and Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD, 20771, USA
¹⁴ Dep. de Física Teórica, Fac. de Ciencias, UAM Campus Cantoblanco, 28049 Madrid, Spain
¹⁵ CEA/IRFU/SAp, AIM UMR 7158, 91191 Gif-sur-Yvette, France
¹⁶ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, UK
¹⁷ School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
¹⁸ UK Astronomy Technology Centre, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 15 July 2011
Accepted: 6 December 2011

Abstract

Context. HD 181327 is a young main sequence F5/F6 V star belonging to the β Pictoris moving group (age  ~12 Myr). It harbors an optically thin belt of circumstellar material at radius  ~90 AU, presumed to result from collisions in a population of unseen planetesimals.

Aims. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio.

Methods. We obtained far-infrared photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory^⋆, complemented by new 3.2 mm observations carried with the ATCA^⋆⋆ array. The geometry of the belt is constrained with newly reduced HST/NICMOS scattered light images that allow the degeneracy between the disk geometry and the dust properties to be broken. We then use the radiative transfer code GRaTeR to compute a large grid of models, and we identify the grain models that best reproduce the spectral energy distribution (SED) through a Bayesian analysis. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo.

Results. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the [OI] and [CII] line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of  ~0.05   M_⊕ (in grains up to 1 mm). We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than  ~17   M_⊕.

Conclusions. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.