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

kappa CrB is a similar to 2.5 Gyr old K1 sub-giant star, with an eccentric exo-Jupiter at similar to 2.8 au and a debris disc at tens of au. We present ALMA Band 6 (1.3 mm) and HST 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 similar to 0.016 M-circle plus, and constrain lower-limit planetesimal sizes as D-max greater than or similar to 1 km and the planetesimal belt mass as M-disc greater than or similar to 1 M-circle plus. We constrain the properties of an outer body causing a linear trend in 17 years of radial velocity data to have a semi-major axis 8 - 66 au and a mass 0.4 - 120 M-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 similar to 0.8 - 0.9) but an inclination (i similar to 30 degrees - 40 degrees) that is inferred to be significantly lower than the i similar to 61 degrees millimetre inclination. The origin of such a discrepancy is unclear, but could be caused by a misalignment in the micron and millimetre sized dust. We place an upper limit on the CO gas mass of M-CO < (4.2 - 13) x 10(-7) M-circle plus, 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.

Automated summary

kappa CrB is a K1 sub-giant star that is approximately 2.5 Gyr old. It has an eccentric exo-Jupiter and a debris disc. Using ALMA Band 6 and HST scattered light images, the researchers observed and modeled the properties of the debris disc, estimated the dust mass, and studied the presence of an outer body causing a linear trend in radial velocity data. The study also analyzed the scattering properties of the dust and placed an upper limit on the CO gas mass.