Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant Kappa CrB

TL;DR

This paper presents the first spatially resolved images of a debris disc around a subgiant star, Kappa CrB, revealing the structure of its planetary system including potential dust belts and planetary companions.

Contribution

It provides the first resolved images of a debris disc around a subgiant and combines these with radial velocity and AO imaging to study the system's planetary and dust structure.

Findings

Dust is broadly distributed, possibly as one wide belt or two narrow belts.

Evidence for a second planetary companion beyond 3AU.

Absence of dust within 20AU may be due to dynamical depletion or collisional erosion.

Abstract

We present Herschel spatially resolved images of the debris disc orbiting the subgiant Kappa CrB. Not only are these the first resolved images of a debris disc orbiting a subgiant, but Kappa CrB is a rare example of an intermediate mass star where a detailed study of the structure of the planetary system can be made, including both planets and planetesimal belt(s). The only way to discover planets around such stars using the radial velocity technique is to observe 'retired' A stars, which are cooler and slower rotators compared to their main-sequence counterparts. A planetary companion has already been detected orbiting the subgiant Kappa CrB, with revised parameters of m sin i = 2.1MJ and apl = 2.8AU (Johnson et al. 2008a). We present additional Keck I HIRES radial velocity measurements that provide evidence for a second planetary companion, alongside Keck II AO imaging that places an upper limit on the mass of this companion. Modelling of our Herschel images shows that the dust is broadly distributed, but cannot distinguish between a single wide belt (from 20 to 220AU) or two narrow dust belts (at around 40 and 165AU). Given the existence of a second planetary companion beyond approximately 3AU it is possible that the absence of dust within approximately 20AU is caused by dynamical depletion, although the observations are not inconsistent with depletion of these regions by collisional erosion, which occurs at higher rates closer to the star.