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

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 $150_{-7}^{+10}$ au with a width ($\sigma$) of 49${}_{-6}^{+7}$ au ($\mathrm{\Delta }R\mathrm{/}R$ = 0.77), at a moderate inclination of 49$\text{fdeg}$9^{+3.3}{-3.7}$\mathrm{.}Duetothelimitedangularresolution,signal-to-noise,andinclinationwehavenoconstraintonthedis{c}^{\mathrm{\prime }}sverticalscaleheight\mathrm{.}Wemodelledthediscemissionwithbothgravitationalandradiationforcesactingonthedustgrains\mathrm{.}Astheinnerbelthasnotbeenspatiallyresolved,wefixeditsradiusandwidthpriortomodellingtheouterbelt\mathrm{.}Wefindastronomicalsilicateisthebestfitforthedustcomposition\mathrm{.}However,wecouldnotrejectpossiblescenarioswherethereareatleast10\mathrm{\%}water-iceinclusions\mathrm{.}CombiningthespatiallyresolvedimagingbyALMAwithnon-detectionatopticalwavelengthsbyHST,weobtainalimitonthescatteringalbedo$ω\leq 0.09$forthedebrisdustintheouterbelt\mathrm{.}Analysisoftheouterbel{t}^{\mathrm{\prime }}sarchitectureinconjunctionwithsimplestirringmodelsplacesamasslimitof$2.3~\pm~0.4 M{\rm Jup}$onacompanioninteriortothebelt($a \leq 78$ au), a factor of two improvement over constraints from high contrast imaging.