The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)

IX. Gas-driven origin for the continuum arc in the debris disc of HD121617

¹ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3493, Santiago, Chile
² Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
³ Center for Interdisciplinary Research in Astrophysics Space Exploration (CIRAS), Universidad de Santiago, Chile
⁴ IRAP, Université de Toulouse, CNRS, CNES, 31400 Toulouse, France
⁵ Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁶ Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
⁷ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Department of Astronomy and Steward Observatory, The University of Arizona, 933 North Cherry Ave, Tucson, AZ 85721, USA
¹⁰ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹¹ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
¹² Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
¹⁴ Instituto de Astrofísica de Canarias, Vía Láctea S/N, La Laguna, 38200 Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna, La Laguna, 38200 Tenerife, Spain
¹⁶ Department of Astronomy, Van Vleck Observatory, Wesleyan University, 96 Foss Hill Dr., Middletown, CT 06459, USA
¹⁷ School of Physics, Trinity College Dublin, the University of Dublin, College Green, Dublin 2, Ireland
¹⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁹ Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720-3411, USA
²⁰ Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218, USA
²¹ Joint ALMA Observatory, Avenida Alonso de Córdova 3107, Vitacura 7630355, Santiago, Chile
²² Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
²³ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
²⁴ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
²⁵ Max-Planck-Insitut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

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Received: 14 August 2025
Accepted: 25 September 2025

Abstract

Context. Debris discs were long considered to be largely gas-free environments, where dynamical evolution is governed primarily by collisional fragmentation, gravitational stirring, and radiative forces. Recent detections of CO molecular line emission in debris discs demonstrate that gas is present, but its abundance and origin are still uncertain. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) observed both the gas and dust of several debris discs at high resolution and revealed a narrow ring of gas and dust in the disc HD 121617, with an asymmetric arc-like feature that is 40% brighter than the rest of the ring.

Aims. An important open question is how representative the estimated CO masses are for the total gas mass in debris discs. We aim to constrain the total gas mass in HD 121617 using numerical models under the assumption that the dust arc is produced by hydrodynamical processes involving the gas.

Methods. We used the hydrodynamical code Dusty FARGO-ADSG, in which dust is modelled as Lagrangian particles. We explored the effects of radiation pressure and dust feedback, as well as of varying the total gas mass on the dynamical evolution of the system. We compared these simulations with observations via radiative transfer calculations.

Results. We find that an unstable gas ring can create a size-dependent radial and azimuthal dust trap. The total gas mass dictates the efficiency of particle trapping as a function of grain size. We find that two of our models, M_gas=50 M_⊕ and M_gas=5 M_⊕, can simultaneously reproduce the observed arc in the ALMA band 7 continuum image and the radial outward offset of the VLT/SPHERE scattered light ring, driven by the combined effects of gas drag and radiation pressure. We further find a conservative lower limit of M_gas>2.5 M_⊕ and a conservative upper limit of M_gas<250 M_⊕.

Conclusions. If the ALMA band 7 asymmetry is caused by gas drag, reconciling the required gas mass with the observed ¹²CO emission suggests the presence of significant amounts of H₂, consistent with the gas being primordial, that is, long-lived remnant material from the protoplanetary disc phase. In this scenario, HD 121617 would represent a hybrid disc, bridging the protoplanetary and debris disc stages. As an arc-shaped emission can alternatively be reproduced by a planet’s gravitational forcing, future observations are crucial to distinguish between these two scenarios.