Can eccentric debris disks be long-lived?

A first numerical investigation and application to ζ²  Reticuli^⋆,⋆⋆

¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: Virginie.Faramaz@obs.ujf-grenoble.fr
² LESIA, Observatoire de Paris, 92195 Meudon, France
³ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Sta. Ma. Tonantzintla, Puebla, Mexico
⁴ Universidad Autónoma de Madrid, Dpto. Física Teórica, Módulo 15, Facultad de Ciencias, Campus de Cantoblanco, 28049 Madrid, Spain
⁵ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁶ Dpt de Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ Aurora Technology B.V., ESA-ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena CA 91109, USA
⁹ NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics, Code 667, Greenbelt MD 20771, USA
¹⁰ Rutherford Appleton Laboratory, Chilton OX11 0QX, UK
¹¹ Department of Physics and Astrophysics, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹² Christian-Albrechts-Universität zu Kiel, Institut für Theoretische Physik und Astrophysik, Leibnizstr. 15, 24098 Kiel, Germany

Received: 9 August 2013
Accepted: 13 December 2013

Abstract

Context. Imaging of debris disks has found evidence for both eccentric and offset disks. One hypothesis is that they provide evidence for massive perturbers, for example, planets or binary companions, which sculpt the observed structures. One such disk was recently observed in the far-IR by the Herschel Space Observatory around ζ²  Reticuli. In contrast with previously reported systems, the disk is significantly eccentric, and the system is several Gyr old.

Aims. We aim to investigate the long-term evolution of eccentric structures in debris disks caused by a perturber on an eccentric orbit around the star. We hypothesise that the observed eccentric disk around ζ²  Reticuli might be evidence of such a scenario. If so, we are able to constrain the mass and orbit of a potential perturber, either a giant planet or a binary companion.

Methods. Analytical techniques were used to predict the effects of a perturber on a debris disk. Numerical N-body simulations were used to verify these results and further investigate the observable structures that may be produced by eccentric perturbers. The long-term evolution of the disk geometry was examined, with particular application to the ζ²  Reticuli system. In addition, synthetic images of the disk were produced for direct comparison with Herschel observations.

Results. We show that an eccentric companion can produce both the observed offsets and eccentric disks. These effects are not immediate, and we characterise the timescale required for the disk to develop to an eccentric state (and any spirals to vanish). For ζ²  Reticuli, we derive limits on the mass and orbit of the companion required to produce the observations. Synthetic images show that the pattern observed around ζ²  Reticuli can be produced by an eccentric disk seen close to edge-on, and allow us to bring additional constraints on the disk parameters of our model (disk flux and extent).

Conclusions. We conclude that eccentric planets or stellar companions can induce long-lived eccentric structures in debris disks. Observations of such eccentric structures thus provide potential evidence of the presence of such a companion in a planetary system. We considered the specific example of ζ²  Reticuli, whose observed eccentric disk can be explained by a distant companion (at tens of AU) on an eccentric orbit (e_p ≳ 0.3).