The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) IV: CO gas imaging and overview

TL;DR

The ARKS IV study delivers high-spectro-spatial-resolution ALMA imaging of CO gas in 18 debris discs, resolving CO in five gas-bearing systems and constraining inner radii and radial distributions. By comparing $^{12}$CO and $^{13}$CO J=3-2 emission, the work derives radially flat isotopologue flux ratios that imply both lines are either optically thick or thin across most discs, and provides CO masses under optically thin and thick assumptions. The CO emission generally extends beyond the dust belt and peaks interior to the dust, requiring shielding or rapid viscous spreading to sustain CO lifetimes; these results inform debates on primordial versus secondary gas origins in debris discs and suggest a diverse set of evolutionary pathways. The study also establishes upper limits for CO in 13 non-detections and places young systems with notable gas content in a broader context of CO line luminosity evolution, highlighting the influence of disc properties and age on gas retention and detectability. Overall, ARKS demonstrates the power of high-resolution CO imaging to dissect gas distribution, optical depth, and potential planet-disc interactions in mature planetary systems.

Abstract

CO gas is detected in a significant number of debris discs, but its origin and evolution remains unclear. Key constraints 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. We used $^{12}$CO and $^{13}$CO J=3-2 line data of 18 ARKS debris belts, 5 of which were already known to host gas, to analyse the spectro-spatial distribution of CO, constrain the gas masses, and to search for gas in the remaining systems. We developed a line-imaging pipeline and produced line cubes for each disc, with a spatial resolution down to $\sim$70 mas and spectral resolution of 26 m s$^{-1}$. Using spectro-spatial shifting and stacking, we produced high signal-to-noise maps, and radial and spectral profiles that reveal the distribution and kinematics of gas in 5 gas-bearing discs. For these discs, we constrained the inner radius of the $^{12}$CO, and found the radial brightness profile of CO peaked interior to the dust ring, but that CO was more radially extended than the dust. We present the first radially resolved $^{12}$CO/$^{13}$CO isotopologue flux ratios in gas-bearing debris discs, which are constant with radius for the majority of systems, indicating $^{12}$CO and $^{13}$CO are both optically thick or thin throughout the discs. We report CO line fluxes/upper limits for all systems and optical depth dependant masses for the 5 gas-bearing systems. Finally, we analysed the $^{12}$CO J=3-2 line luminosities for the ARKS debris discs and 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.

Figures (8)

• Figure 1: $^{12}$CO (left) and $^{13}$CO (right) emission of the five gas-bearing disks in ARKS, HD 9672, HD 32297, HD 121617, HD 131488, and HD 131835 (from top to bottom). The first and fourth column in each row shows the spectrally integrated-intensity map of each disc for $^{12}$CO and $^{13}$CO, respectively. The minimum line intensity of these integrated-intensity maps was set at 0 mJy km s$^{-1}$ beam$^{-1}$. The second and fifth columns shows the peak velocity map of each disc for $^{12}$CO and $^{13}$CO, respectively. Each is centred on the stellar velocity of the systems, and any pixel outside of the velocity range shown in the colour bar is set to black. The peak-intensity (brightness temperature) map of each disc is shown in the third and sixth column for $^{12}$CO and $^{13}$CO, respectively. The minimum brightness temperature shown in the colour bar is the mean of the emission far from the discs. The ellipse in the lower left corners of the maps is the synthesized beam of the observations. The white scale bar in the bottom right corners indicates 50 au.
• Figure 2: Left column: Spatially integrated spectrum of the $^{12}$CO(3-2) (red) and $^{13}$CO(3-2) (blue) emission from HD 9672, HD 32297, HD 121617, HD 131488, and HD 131835 in the barycentric velocity frame. The $^{12}$CO spectra are additionally shown smoothed with a running mean (window size = 20), chosen to remove noise variation while preserving spectral features. Right column: radial profiles for the same discs. Radial profiles for HD 9672, HD 121617 and HD 131488 were created by azimuthally integrating the emission in each disc. The radial profiles for HD 32297 and HD 131488 (marked by *) are surface brightness profiles as a function of projected separation from the star along the major axis (rather than as a function of radius). For HD 32297, the radial distribution varies for each side of the discs so both sides are shown, in both $^{12}$CO and $^{13}$CO. For HD 131488, there is no significant differences between both sides of the disc, so an average radial profile is shown. The black radial profiles show the CLEAN dust profiles Yinuo_arks. In each panel, the minimum value of $R$ corresponds to the first resolution element of the observations. The red, blue and black horizontal bars indicate the resolution of the observations.
• Figure 3: Radial profiles of $^{12}$CO/$^{13}$CO intensity ratios for HD 9672, HD 32297, HD 121617, HD 131488, and HD 131835 (blue). The $^{12}$CO radial profile is overplotted in red to show the gas distribution in each system. The $^{12}$CO/$^{13}$CO ratios for HD 32297 and HD 131488 (marked by an asterisk) were created from the average of the non-deprojected on-sky radial profiles. In each panel, the minimum value of R corresponds to the first resolution element of the observations. The black bar indicates the resolution of the observations.
• Figure 4: $^{12}$CO spatially integrated spectrum of HD 121617(blue) and its spectro-spatially stacked (S.S.) spectrum (red). The dotted red lines indicate the velocities in which integration was carried out to calculate the error on the line flux measurement. $V_*$ represents the stellar velocity of HD121617, determined by fitting a Gaussian profile to the spectro-spatially shifted spectrum.
• Figure 5: Line flux of $^{12}$CO (J=3-2) scaled to 100 pc as a function of the stellar age. The pink points show $^{12}$CO (J=3-2) fluxes measured in this work, the green points show archival $^{12}$CO (J=3-2) fluxes, and the purple points show archival $^{12}$CO (J=2-1) fluxes that were scaled by the average $^{12}$CO J=3-2/J=2-1 ratio of all discs for which both lines were observed (1.67). The archival sources and the corresponding references are listed in Table \ref{['tab:archival_linefluxes']}. The circles represent resolved data points, and the triangles represent upper limits on $^{12}$CO.