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

Abstract

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.

Figures (21)

• Figure 1: $^{12}\textrm{CO}$ emission for HD 121617. Left: Velocity integrated intensity map created by shifting the spectral emission and integrating along the velocity axis for all channels with emission above 4$\sigma$. Middle: Peak intensity map generated using the quadratic method with bettermoments. Right: Linewidth FWHM map produced by dividing the velocity integrated intensity map by the peak intensity map and converting it to FWHM, assuming a Gaussian line profile shape. The beam size is shown in the lower-left corner of each panel. The horizontal white bar indicates 50 au. The corresponding maps for $^{13}\textrm{CO}$ can be found in Appendix \ref{['sec:addional_figures']}.
• Figure 2: Normalised radial profile for $^{12}\textrm{CO}$ (blue) and $^{13}\textrm{CO}$ (red) produced using gofish and continuum (green) produced using frank. These radial profiles are not de-convolved and, therefore, might not be directly comparable in width. The lower x-axis limit in each radial profile is set at the first resolved resolution element of the $^{12}\textrm{CO}$ observation. The blue, red, and green bars show the resolution of the observations.
• Figure 3: Local line profiles for $^{12}\textrm{CO}$ (left panel) and $^{13}\textrm{CO}$ (right panel) as a function of distance from the star. The error bars for $^{12}\textrm{CO}$ in the left panel (shaded regions) and for $^{13}\textrm{CO}$ in the right panel (error bars) have been re-scaled by constant factors $f_{12}$ and $f_{13}$, respectively, as fitted in Section \ref{['sec:1DResults']}. These errors are small and only marginally visible.
• Figure 4: Local line profiles for $^{12}\textrm{CO}$ (blue) and $^{13}\textrm{CO}$ (red) created from an annulus extracted at the peak gas surface density ($0\farcs61$-$0\farcs63$). Best-fit models (dashed lines) are generated using best-fit values from Table \ref{['table:best_fit_model']}. Top left: Spectral resolution for $^{12}\textrm{CO}$ (blue) and $^{13}\textrm{CO}$ (red). Error bars for $^{12}\textrm{CO}$ (blue) and $^{13}\textrm{CO}$ (red) have been re-scaled by the fitted factor f$_{12}$ and f$_{13}$.
• Figure 5: Comparison of FWHM (m s$^{-1}$) for $^{12}\textrm{CO}$ ALMA data (blue line) and the RADMC-3D optically thin model (dashed red line), where the FWHM is derived by fitting a Gaussian to each spectrum (assuming azimuthal sections of 15$^\circ$ for the data) using an MCMC approach for both the model and data. For both the model and the data, an annulus between $0\farcs61$ and $0\farcs63$ (corresponding to the disc’s peak radial surface density) is extracted. The error bars for the $^{12}\textrm{CO}$ ALMA data (shaded regions) have been re-scaled by a constant factor, which was fitted for.