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

Abstract

$\hat{I}º$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 ($0.6\hat{I}¼$m) images, demonstrating $\hat{I}º$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 $\sim 0.016M\oplus$, and constrain lower-limit planetesimal sizes as $D_{\max }>1$km and the planetesimal belt mass as $M_{\mathrm{disc}}>1M_{\oplus }$. 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 $(0.4-120)M_{\mathrm{Jup}}$. 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 $M_{\mathrm{CO}}<(4.2-13)\times 10^{-7}{M}_{\oplus }$, and show this to be consistent with levels expected from planetesimal collisions, or from CO-ice sublimation as $\hat{I}º$CrB begins its giant branch ascent.

Accepted for publication in MNRAS. 21 pages, 14 figures