The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)

IV. CO gas imaging and overview

¹ School of Physics, Trinity College Dublin, the University of Dublin, College Green, Dublin 2, Ireland
² Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
³ Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁴ Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
⁵ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3493, Santiago, Chile
⁶ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
⁷ Center for Interdisciplinary Research in Astrophysics Space Exploration (CIRAS), Universidad de Santiago, Santiago, Chile
⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Joint ALMA Observatory, Avenida Alonso de Córdova 3107, Vitacura 7630355, Santiago, Chile
¹⁰ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
¹¹ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
¹² Department of Astronomy, Van Vleck Observatory, Wesleyan University, 96 Foss Hill Dr., Middletown, CT 06459, USA
¹³ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
¹⁴ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹⁵ Max-Planck-Insitut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Academia Sinica Institute of Astronomy and Astrophysics, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sect. 4, Roosevelt Rd, Taipei 106319, Taiwan
¹⁷ Herzberg Astronomy & Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E9, Canada
¹⁸ Department of Physics & Astronomy, University of Victoria, 3800 Finnerty Rd, Victoria, BC V8P 5C2, Canada
¹⁹ Center for Astrophysics I Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
²⁰ Department of Astronomy and Steward Observatory, The University of Arizona, 933 North Cherry Ave, Tucson, AZ 85721, USA
²¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²² Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720-3411, USA
²³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
²⁴ Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218, USA
²⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
²⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²⁷ Instituto de Astrofísica de Canarias, Vía Láctea S/N, La Laguna, 38200 Tenerife, Spain
²⁸ Departamento de Astrofísica, Universidad de La Laguna, La Laguna, 38200 Tenerife, Spain

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Received: 25 July 2025
Accepted: 24 September 2025

Abstract

Context. CO gas is detected in a significant number (~20) of debris discs (exoKuiper belts), but understanding its origin and evolution remains elusive. Crucial pieces of evidence are its mass and spectro-spatial distribution, which are coupled through optical depth and have only been analysed at low to moderate resolution so far. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) is the first ALMA large program to target debris discs at high spectro-spatial resolution.

Aims. We used ¹²CO and ¹³CO J=3-2 line data of 18 debris belts observed by ARKS, 5 of which were already known to be gas-bearing, in order to analyse the spectro-spatial distribution of CO and constrain the gas mass in discs that were known to host gas previously, and to search for gas in the remaining 13 discs without previous CO detections.

Methods. We developed a line-imaging pipeline for ARKS CO data with a high spectro-spatial resolution. Using this tool, we produced line cubes for each of the ARKS targets, with a spatial resolution down to about 70 mas and a spectral resolution of 26 m s⁻¹. We used spectro-spatial shifting and stacking techniques to produce a gallery of maps with the highest possible signal-to-noise ratio (S/N) and with radial and spectral profiles that reveal the distribution and kinematics of gas in five gas-bearing discs at unprecedented detail.

Results. For each of the five gas-bearing discs (HD 9672/49 Ceti, HD 32297, HD 121617, HD 131488, and HD 131835), we constrained the inner radius of the ¹²CO (r_min ~ 3-68 au), and we found that the radial brightness profile of CO peaked interior to the dust ring, but that CO was also more radially extended than the dust. In a second-generation scenario, this would require significant shielding of CO that would allow it to viscously spread to the observed widths. We present the first radially resolved ¹²CO/¹³CO isotopologue flux ratios in five gas-bearing debris discs and found them to be constant with radius for the majority (four out of five) of systems. This indicates that ¹²CO and ¹³CO are both optically thick or optically thin throughout the discs. We report CO line fluxes or upper limits for all systems and optical depth dependant masses for the five systems with detected CO. Finally, we analysed the ¹²CO J=3-2 line luminosities for a range of ARKS debris discs and for debris discs from the literature. We confirm that gas is mostly detected in young systems. However, the high scatter seen in young/high fractional luminosity systems indicates no trend within the systems with detected gas. This could be caused by different system properties and/or evolution pathways.