A peculiar class of debris disks from Herschel/DUNES^⋆

A steep fall off in the far infrared

¹ UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: steve.ertel@obs.ujf-grenoble.fr
² Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 15, 24098 Kiel, Germany
³ Dpt. Física Teórica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
⁴ Astrophysikalisches Institut und Universitätssternwarte, Friedrich-Schiller-Universität, Schillergäßchen 2-3, 07745 Jena, Germany
⁵ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du Six Août 17, 4000 Sart Tilman, Belgium
⁶ NASA Herschel Science Center, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁷ Department of Astrophysics, Centre for Astrobiology (CAB, CSIC-INTA), ESAC Campus, 28691 PO Box 78, Villanueva de la Cañada, Madrid, Spain
⁸ European Space Observatory, Alonso de Cordova 3107, Vitacura Casilla 19001, Santiago 19, Chile
⁹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁰ UNINOVA-CA3, Campus da Caparica, Quinta da Torre, Monte de Caparica, 2825-149 Caparica, Portugal
¹¹ School of Physics and Astronomy, St Andrews University, North Haugh, St Andrews, Fife KY16 9SS, UK
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
¹⁴ ESA-ESAC Gaia SOC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁵ ESA Astrophysics & Fundamental Physics Missions Division, ESTEC/SRE-SA, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁶ Code 667, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
¹⁷ Department of Physics and Astrophysics, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁸ Rutherford Appleton Laboratory, Chilton OX11 0QX, UK

Received: 12 September 2011
Accepted: 19 March 2012

Abstract

Context. The existence of debris disks around old main sequence stars is usually explained by continuous replenishment of small dust grains through collisions from a reservoir of larger objects.

Aims. We present photometric data of debris disks around HIP 103389 (HD 199260), HIP 107350 (HN Peg, HD 206860), and HIP 114948 (HD 219482), obtained in the context of our Herschel open time key program DUNES (DUst around NEarby Stars).

Methods. We used Herschel/PACS to detect the thermal emission of the three debris disks with a 3σ sensitivity of a few mJy at 100   μm and 160   μm. In addition, we obtained Herschel/PACS photometric data at 70   μm for HIP 103389. These observations are complemented by a large variety of optical to far-infrared photometric data. Two different approaches are applied to reduce the Herschel data to investigate the impact of data reduction on the photometry. We fit analytical models to the available spectral energy distribution (SED) data using the fitting method of simulated thermal annealing as well as a classical grid search method.

Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths  ≥70   μm. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Using grain compositions that have been applied successfully for modeling of many other debris disks, our modeling reveals that such a steep decrease of the SEDs in the long wavelength regime is inconsistent with a power-law exponent of the grain size distribution  −3.5 expected from a standard equilibrium collisional cascade. In contrast, a steep grain size distribution or, alternatively an upper grain size in the range of few tens of micrometers are implied. This suggests that a very distinct range of grain sizes would dominate the thermal emission of such disks. However, we demonstrate that the understanding of the data of faint sources obtained with Herschel is still incomplete and that the significance of our results depends on the version of the data reduction pipeline used.

Conclusions. A new mechanism to produce the dust in the presented debris disks, deviations from the conditions required for a standard equilibrium collisional cascade (grain size exponent of  −3.5), and/or significantly different dust properties would be necessary to explain the potentially steep SED shape of the three debris disks presented.