High-resolution ALMA and HST imaging of κCrB: a broad debris disc around a post-main-sequence star with low-mass companions

$\kappa \,$ CrB is an ~2.5 Gyr old K1 sub-giant star, with an eccentric exo-Jupiter at ~2.8 au and a debris disc at tens of au. We present ALMA (Atacama Large Millimetre/submillimetre Array) Band 6 (1.3 mm) and Hubble Space Telescope scattered light (0.6$\, \mu$m) images, demonstrating $\kappa \,$CrB's broad debris disc, covering an extent $50\!-\!180\,$au in the millimetre (peaking at 110 au), and $51\!-\!280\,$au in scattered light (peaking at 73 au). By modelling the millimetre emission, we estimate the dust mass as ${\sim }0.016\, {\rm M}_\oplus$, and constrain lower-limit planetesimal sizes as $D_{\rm {max}}{\gtrsim }1\,$km and the planetesimal belt mass as $M_{\rm {disc}}{\gtrsim }1\, {\rm M}_\oplus$. We constrain the properties of an outer body causing a linear trend in 17 yr of radial velocity data to have a semimajor axis 8-66 au and a mass $0.4\!-\!120\, M_{\rm {Jup}}$. There is a large inner cavity seen in the millimetre emission, which we show is consistent with carving by such an outer massive companion with a string of lower mass planets. Our scattered light modelling shows that the dust must have a high anisotropic scattering factor (g ~ 0.8-0.9) but an inclination (i ~ 30°-40°) that is inferred to be significantly lower than the i ~ 61° millimetre inclination. The origin of such a discrepancy is unclear, but could be caused by a misalignment in the micrometre- and millimetre-sized dust. We place an upper limit on the CO gas mass of $M_{\rm {CO}}{\lt }(4.2\!-\!13) \times 10^{-7}\, {\rm M}_\oplus$, and show this to be consistent with levels expected from planetesimal collisions, or from CO-ice sublimation as $\kappa \,$CrB begins its giant branch ascent.