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

CO gas has been detected in $\sim$20 debris discs. We present ALMA observations of the CO-rich HD 121617 debris disc from the ARKS survey. Using high-resolution Band 7 observations of $^{12}CO \ J=3-2$, we analyse local CO line profiles to investigate optical depth, CO mass, and temperature. Spectra are aligned and stacked in concentric annuli to produce local line profiles. The resulting profiles are Gaussian-shaped and broadened by Keplerian shear. The line profiles are modelled using both a simplified toy model and a RADMC-3D model including projection effects and Keplerian shear. Fitting the RADMC-3D model to the $^{13}$CO data, we find that an optically thick model with a temperature of 38 K and a CO mass of $2 \times 10^{-3}$ M$_{\oplus}$ reproduces the observations. The model reproduces the enhanced emission at orbital azimuths of $\sim \pm45^{\circ}$ and $\pm135^{\circ}$, forming an X-shaped structure in the velocity-integrated intensity map, as well as the broader $^{12}$CO linewidth relative to $^{13}$CO. Scaling the model by the ISM abundance ratio ($\sim$77) also reproduces the $^{12}$CO data, though high optical depths and model assumptions limit mass constraints. We find that azimuthally averaged local line profiles appear Gaussian regardless of optical depth, cautioning against their use for distinguishing optically thin and thick emission. We constrain the mean molecular weight to $12.6_{-1.1}^{+1.3}$, dependent on model assumptions. Our $^{13}$CO results suggest that C$^{18}$O may also be optically thick in CO-rich debris discs and that the mean molecular weight is significantly higher than if H$_2$ were the dominant gas species, suggesting a non-primordial composition.