A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved with Herschel^⋆

¹ Observatoire de Paris, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: jean-francois.lestrade@obspm.fr
² Herzberg Institute of Astrophysics (HIA), National Research Council of Canada, Victoria, BC, Canada
³ UK Astronomy Technology Centre (UKATC), Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ Institute of Astronomy (IoA), University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁶ School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK
⁷ Department of Astrophysics, Center for Astrobiology, Ctra. de Ajalvir, km 4, Torrejťon de Ardoz, 28850 Madrid, Spain
⁸ Dept. of Physics & Astronomy, University of Victoria, Elliott Building, 3800 Finnerty Rd, Victoria, BC, V8P 5C2, Canada
⁹ ALMA JAO, Av. El Golf 40 – Piso 18, Las Condes, Santiago, Chile
¹⁰ Astronomy Department, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, USA
¹¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
¹² SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA
¹³ Astronomy Department, University of Texas at Austin, 1 University Station C1400, Austin, TX, USA
¹⁴ Department of Astrophysics and Optics, School of Physics, University of New South Wales, 2052 Sydney, Australia
¹⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 3 September 2012
Accepted: 5 November 2012

Abstract

Debris disks have been found primarily around intermediate and solar mass stars (spectral types A–K) but rarely around low mass M-type stars. We have spatially resolved a debris disk around the remarkable M3-type star GJ 581 hosting multiple planets using deep PACS images at 70, 100 and 160 μm as part of the DEBRIS Program on the Herschel Space Observatory. This is the second spatially resolved debris disk found around an M-type star, after the one surrounding the young star AU Mic (12 Myr). However, GJ 581 is much older (2–8 Gyr), and is X-ray quiet in the ROSAT data. We fit an axisymmetric model of the disk to the three PACS images and found that the best fit model is for a disk extending radially from 25 ± 12 AU to more than 60 AU. Such a cold disk is reminiscent of the Kuiper belt but it surrounds a low mass star (0.3 M_⊙) and its fractional dust luminosity L_dust/L_∗ of ~ 10^-4 is much higher. The inclination limits of the disk found in our analysis make the masses of the planets small enough to ensure the long-term stability of the system according to some dynamical simulations. The disk is collisionally dominated down to submicron-sized grains and the dust cannot be expelled from the system by radiation or wind pressures because of the low luminosity and low X-ray luminosity of GJ 581. We suggest that the correlation between low-mass planets and debris disks recently found for G-type stars also applies to M-type stars. Finally, the known planets, of low masses and orbiting within 0.3 AU from the star, cannot dynamically perturb the disk over the age of the star, suggesting that an additional planet exists at larger distance that is stirring the disk to replenish the dust.