Long-wavelength observations of debris discs around sun-like stars^*

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany e-mail: roccata@mpia.de
² University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstrasse 15, 24098 Kiel, Germany
³ Research and Scientific Support Department, ESA/ESTEC, 2201 AZ Noordwijk, The Netherlands
⁴ California Institute of Technology, Department of Astronomy, MS 105-24, Pasadena, CA 91125, USA
⁵ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁶ Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 169-506, Pasadena, CA 91109, USA

Received: 23 September 2008
Accepted: 2 February 2009

Abstract

Context. Tracing the evolution of debris discs is essential for our understanding of the architecture of planetary system. Even though the evolution of their inner discs has been recently studied with the Spitzer Space Telescope at mid- to far-infrared wavelengths, the outer discs are best characterised by sensitive millimetre observations.

Aims. The goal of our study is to understand the evolution timescale of circumstellar debris discs, and the physical mechanisms responsible for such evolution around solar-type stars. In addition, we make a detailed characterisation of the detected debris discs.

Methods. Two deep surveys of circumstellar discs around solar-type stars at different ages were carried out at 350 μm with the CSO and at 1.2 mm with the IRAM 30-m telescope. The dust disc masses were computed from the millimetre emission, where the discs are optically thin. Theoretically, the mass of the disc is expected to decrease with time. To test this hypothesis, we performed the generalised Kendall's tau correlation and three different two-sample tests. A characterisation of the detected debris discs has been obtained by computing the collision and Poynting-Robertson timescales and by modelling the spectral energy distribution.

Results. The Kendall's tau correlation yields a probability of 76% that the mass of debris discs and their age are correlated. Similarly, the three two-sample tests give a probability between 70 and 83% that younger and older debris systems belong to different parent populations in terms of dust mass. We detected submillimetre/millimetre emission from six debris discs, enabling a detailed SED modelling.

Conclusions. Our results on the correlation and evolution of dust mass as a function of age are conditioned by the sensitivity limit of our survey. Deeper millimetre observations are needed to confirm the evolution of debris material around solar-like stars. In the case of the detected discs, the comparison between collision and Poynting-Robertson timescales supports the hypothesis that these discs are dominated by collisions. All detected debris disc systems show the inner part evacuated from small micron-sized grains.