The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)

VII. Optically thick gas with broad CO gaussian local line profiles in the HD 121617 disc

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 National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
⁴ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
⁵ Department of Astronomy, Van Vleck Observatory, Wesleyan University, 96 Foss Hill Dr., Middletown, CT 06459, USA
⁶ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3493, Santiago, Chile
⁷ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
⁸ Center for Interdisciplinary Research in Astrophysics Space Exploration (CIRAS), Universidad de Santiago, Chile
⁹ Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹⁰ Academia Sinica Institute of Astronomy and Astrophysics, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sect. 4, Roosevelt Rd, Taipei 106319, Taiwan
¹¹ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
¹² Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
¹³ Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218, USA
¹⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
¹⁵ Max-Planck-Insitut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁸ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹⁹ Joint ALMA Observatory, Avenida Alonso de Córdova 3107, Vitacura 7630355, Santiago, Chile
²⁰ Instituto de Astrofísica de Canarias, Vía Láctea S/N, La Laguna, 38200, Tenerife, Spain
²¹ Departamento de Astrofśica, Universidad de La Laguna, La Laguna, 38200, Tenerife, Spain
²² Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720-3411, USA

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Received: 23 July 2025
Accepted: 1 December 2025

Abstract

Context. CO gas has been detected in ~20 debris discs, typically classified as CO-poor or CO-rich. We present observations of the CO-rich HD 121617 debris disc as part of the ALMA survey to Resolve exoKuiper belt Substructures (ARKS).

Aims. We model local CO line profiles in the HD 121617 debris disc to investigate optical depth, CO mass, and temperature. HD 121617 is a well-suited ARKS target due to its previously detected CO emission and moderate inclination, reducing the effect of Keplerian shear.

Methods. Using high-resolution ALMA Band 7 observations of ¹²CO J=3–2 (26 m s⁻¹, 0_⋅′′ 1), we create local line profiles by aligning and stacking spectra in concentric annuli of 0_⋅′′ 02 width. These profiles are modelled with both a toy model and a RADMC-3D model that includes projection effects and Keplerian shear.

Results. The resulting local profiles are Gaussian-shaped and broad due to the effect of Keplerian shear. Fitting a RADMC-3D model to the ¹³CO data, we find that an optically thick model (temperature of 38 K and mass of 2 × 10⁻³ M_⊕) reproduces the data, particularly the enhanced intensity at orbital azimuths of ~±45° and ±135°, which forms an X-shape in the velocity integrated intensity map, as well as the broader ¹²CO linewidth compared to ¹³CO. Scaling this model by the ISM abundance ratio (~77) also reproduces the ¹²CO data, but high optical depths and model assumptions limit mass constraints.

Conclusions. Keplerian shear causes azimuthally averaged line profiles to appear Gaussian regardless of optical depth; therefore, we caution against using the local line profiles to distinguish between optically thin and thick emission. We constrain the mean molecular weight to 12.6_−1.1^+1.3, dependent on model assumptions. Although model dependent, our ¹³CO results indicate that C¹⁸O might also be optically thick in CO-rich debris discs, contrary to previous assumptions, and that the mean molecular weight is significantly higher than if H₂ were the dominant gas species, suggesting a non-primordial composition.