Herschel images of Fomalhaut^⋆

An extrasolar Kuiper belt at the height of its dynamical activity

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: bram@ster.kuleuven.be
² Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
³ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ Afdeling Sterrenkunde, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands
⁵ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, EH9 3HJ, UK
⁶ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
⁷ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁸ Department of Physics and Astronomy, University College London, Gower St, London WC1E 6BT, UK
⁹ ALMA, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁰ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
¹¹ ESA Research and Science Support Department, ESTEC/SRE-S, Keplerlaan 1, 2201AZ, Noordwijk, The Netherlands
¹² School of Physics and Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
¹³ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹⁴ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁵ Department of Astronomy, University of Texas, 1 University Station C1400, Austin, TX 78712, USA
¹⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
¹⁸ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ Earth and Space Sciences, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
²⁰ Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada
²¹ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
²² Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Cantoblanco, Spain

Received: 5 December 2011
Accepted: 29 February 2012

Abstract

Context. Fomalhaut is a young (2 ± 1 × 10⁸ years), nearby (7.7 pc), 2 M_⊙ star that is suspected to harbor an infant planetary system, interspersed with one or more belts of dusty debris.

Aims. We present far-infrared images obtained with the Herschel Space Observatory with an angular resolution between 5.7′′ and 36.7′′ at wavelengths between 70 μm and 500 μm. The images show the main debris belt in great detail. Even at high spatial resolution, the belt appears smooth. The region in between the belt and the central star is not devoid of material; thermal emission is observed here as well. Also at the location of the star, excess emission is detected. We aim to construct a consistent image of the Fomalhaut system.

Methods. We use a dynamical model together with radiative-transfer tools to derive the parameters of the debris disk. We include detailed models of the interaction of the dust grains with radiation, for both the radiation pressure and the temperature determination. Comparing these models to the spatially resolved temperature information contained in the images allows us to place strong constraints on the presence of grains that will be blown out of the system by radiation pressure. We use this to derive the dynamical parameters of the system.

Results. The appearance of the belt points toward a remarkably active system in which dust grains are produced at a very high rate by a collisional cascade in a narrow region filled with dynamically excited planetesimals. Dust particles with sizes below the blow-out size are abundantly present. The equivalent of 2000 one-km-sized comets are destroyed every day, out of a cometary reservoir amounting to 110 Earth masses. From comparison of their scattering and thermal properties, we find evidence that the dust grains are fluffy aggregates, which indicates a cometary origin. The excess emission at the location of the star may be produced by hot dust with a range of temperatures, but may also be due to gaseous free-free emission from a stellar wind.