The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)

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 12 CO 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 13 CO 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 12 CO linewidth compared to 13 CO. Scaling this model by the ISM abundance ratio (~77) also reproduces the 12 CO 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 13 CO results indicate that C 18 O 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 H 2 were the dominant gas species, suggesting a non-primordial composition.