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

TL;DR

This study uses ALMA and HST imaging to reveal a broad debris disc around the post-main sequence star $ppa$CrB, constraining its structure, dust properties, and potential planetary influences, and examining gas content and dust alignment.

Contribution

First detailed imaging of $ppa$CrB's debris disc across multiple wavelengths, revealing its extent, dust mass, and potential planetary companions with implications for disc evolution.

Findings

Debris disc extends from 50 to 180 au in millimetre and 51 to 280 au in scattered light.

Estimated dust mass is approximately 0.016 Earth masses.

Outer companion likely causes the large inner cavity and has a semi-major axis of 8-66 au.

Abstract

$\kappa$CrB is a ${\sim}2.5\,$Gyr old K1 sub-giant star, with an eccentric exo-Jupiter at ${\sim}2.8\,$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 ${\sim}0.016\,M{\oplus}$, and constrain lower-limit planetesimal sizes as $D_{\rm{max}}{>}1\,$km and the planetesimal belt mass as $M_{\rm{disc}}{>}1\,M_{\oplus}$. 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 $(0.4{-}120)\,M_{\rm{Jup}}$. 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 ($g{\sim}0.8{-}0.9$) but an inclination ($i{\sim}30{-}40\,$degrees) that is inferred to be significantly lower than the $i{\sim}61\,$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 $M_{\rm{CO}}{<}(4.2{-}13){\times}10^{-7}\,M_{\oplus}$, 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.