The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)

Context . ExoKuiper belts around young A-type stars often host CO gas, whose origin is still unclear. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) includes six of these gas-bearing belts, to characterise their dust and gas distributions and investigate the gas origin. Aims . As part of ARKS, we observed the gas-rich system HD 121617 with a 0 ⋅ ′′ 12 (14 au) resolution and discovered an arc of enhanced dust density. In this paper, we analyse in detail the dust and gas distributions and the gas kinematics of this system. Methods . We extracted radial and azimuthal profiles of the dust (in the millimetre and near-infrared) and gas emission ( 12 CO and 13 CO) from reconstructed images. To constrain the morphology of the arc, we fitted an asymmetric model to the dust emission. To characterise the gas kinematics, we fitted a Keplerian model to the velocity map and extracted the gas azimuthal velocity profile by deprojecting the data. Results . We find that the dust arc is narrow (1–5 au wide at a radius of 75 au), azimuthally extended with a full width at half maximum of ~90°, and asymmetric; the emission is more azimuthally compact in the direction of the system’s rotation, and represents 13% of the total dust mass (0.2 M ⊕ ). From analysis of the scattered light and CO images, we conclude that the arc is much less pronounced or absent for small grains and gas. Finally, we find strong non-Keplerian azimuthal velocities at the inner and outer wings of the ring, as was expected due to strong pressure gradients. Conclusions . The dust arc resembles the asymmetries found in protoplanetary discs, often interpreted as the result of dust trapping in vortices. If the gas disc mass is high enough (≳20 M ⊕ , requiring a primordial gas origin), both the radial confinement of the ring and the azimuthal arc may result from dust grains responding to gas drag. Alternatively, it could result from planet-disc interactions via mean motion resonances. Further studies should test these hypotheses and may provide a dynamical gas mass estimate in this CO-rich exoKuiper belt.