ALMA millimetre-wavelength imaging of HD 138965: New constraints on the debris dust composition and presence of planetary companions

TL;DR

This study uses ALMA millimetre imaging to characterize the debris disc around HD 138965, constraining its size, composition, and potential planetary companions, revealing a silicate-rich dust composition and limiting possible planetary mass.

Contribution

First detailed millimetre imaging of HD 138965's debris disc, providing new constraints on its structure, dust composition, and limits on interior planetary companions.

Findings

Disc extends to 150 au with a width of 49 au.

Dust is best fit by astronomical silicate with possible water-ice inclusions.

Limits on interior planetary mass are improved to 2.3 Jupiter masses.

Abstract

HD 138965 is a young A type star and member of the nearby young Argus association. This star is surrounded by a broad, bright debris disc with two temperature components that was spatially resolved at far-infrared wavelengths by Herschel. Here we present ALMA millimetre-wavelength imaging of the cool outer belt. These reveal its radial extent to be $150^{+10}_{-7}$ au with a width ($\sigma$) of 49$^{+7}_{-6}$ au (${\Delta}R/R$ = 0.77), at a moderate inclination of 49$\fdeg$9^{+3.3}_{-3.7}$. Due to the limited angular resolution, signal-to-noise, and inclination we have no constraint on the disc's vertical scale height. We modelled the disc emission with both gravitational and radiation forces acting on the dust grains. As the inner belt has not been spatially resolved, we fixed its radius and width prior to modelling the outer belt. We find astronomical silicate is the best fit for the dust composition. However, we could not reject possible scenarios where there are at least 10 \% water-ice inclusions. Combining the spatially resolved imaging by ALMA with non-detection at optical wavelengths by HST, we obtain a limit on the scattering albedo $\omega \leq 0.09$ for the debris dust in the outer belt. Analysis of the outer belt's architecture in conjunction with simple stirring models places a mass limit of $2.3~\pm~0.4 M_{\rm Jup}$ on a companion interior to the belt ($a \leq 78$ au), a factor of two improvement over constraints from high contrast imaging.