Letter to the Editor

q Eridani: a solar-type star with a planet and a dust belt^*

¹ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden e-mail: rene.liseau@chalmers.se
² European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: crisache@eso.org
³ Stockholm Observatory, AlbaNova University Center, Roslagstullsbacken 21, 106 91 Stockholm, Sweden e-mail: [alexis;ricky;olofsson@astro.su.se];philippe.thebault@obspm.fr
⁴ Departamento de Física Teórica, C-XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain e-mail: carlos.eiroa@uam.es
⁵ ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands e-mail: malcolm.fridlund@esa.int;gpilbratt@rssd.esa.int

Received: 19 December 2007
Accepted: 1 February 2008

Abstract

Context.Far-infrared excess emission from main-sequence stars is due to dust produced by orbiting minor bodies. In these disks, larger bodies, such as planets, may also be present and the understanding of their incidence and influence currently presents a challenge.

Aims.Only very few solar-type stars exhibiting an infrared excess and harbouring planets are known to date. Indeed, merely a single case of a star-planet-disk system has previously been detected at submillimeter (submm) wavelengths. Consequently, one of our aims is to understand the reasons for these poor statistics, i.e., whether these results reflected the composition and/or the physics of the planetary disks or were simply due to observational bias and selection effects. Finding more examples would be very significant.

Methods.The selected target, q¹ Eri, is a solar-type star, which was known to possess a planet, q¹ Eri b, and to exhibit excess emission at IRAS wavelengths, but had remained undetected in the millimeter regime. Therefore, submm flux densities would be needed to better constrain the physical characteristics of the planetary disk. Consequently, we performed submm imaging observations of q¹ Eri.

Results.The detected dust toward q¹ Eri at 870 μm exhibits the remarkable fact that the entire SED, from the IR to mm-wavelengths, is fit by a single-temperature blackbody function (60 K). This would imply that the emitting regions are confined to a narrow region (ring) at radial distances much larger than the orbital distance of q¹ Eri b, and that the emitting particles are considerably larger than some hundred micron. However, the 870 μm source is extended, with a full-width-half-maximum of roughly 600 AU. Therefore, a physically more compelling model also invokes a belt of cold dust (17 K), located at 300 AU from the star and about 60 AU wide.

Conclusions.The minimum mass of 0.04  (3 M_Moon) of 1 mm-size icy ring-particles is considerable, given the stellar age of  1 Gyr. These big grains form an inner edge at about 25 AU, which may suggest the presence of an unseen outer planet (q¹ Eri c).