The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) I: Motivation, sample, data reduction, and results overview

TL;DR

ARKS is the first ALMA large program dedicated to debris discs, assembling high-resolution millimeter imaging of 24 exoKuiper belts to map dust radial and vertical structure and to characterize CO gas content. By combining tailored observing strategies, meticulous data reduction, and forward-modeling with a diverse set of dynamical scenarios, ARKS reveals a broad diversity of substructures, including multiple rings, narrow peaks within wider belts, and non-Gaussian vertical profiles, alongside widespread asymmetries and complex gas dynamics. The initial results (ARKS II–X) show that dust and gas distributions can diverge and that gas can influence dust dynamics, with several belts exhibiting non-Keplerian kinematics and possible vortex-like features, implying ongoing planet–disc interactions or gas-driven processes. The public data release and multi-paper series lay a foundation for interpreting debris-disc architectures in the context of planet formation, stirring mechanisms, and gas evolution, with significant implications for future observations by JWST and ELT-class facilities.

Abstract

The outer regions of planetary systems host dusty debris discs analogous to the Kuiper belt (exoKuiper belts), which provide crucial constraints on planet formation and evolution processes. ALMA dust observations have revealed a great diversity, and that some belts contain CO gas, whose origin and implications are uncertain. Most of this progress, however, has been limited by low-resolution observations. We conducted the first ALMA large programme dedicated to debris discs: the ALMA survey to Resolve exoKuiper belt Substructures (ARKS). We selected the 24 most promising belts to constrain their detailed radial and vertical structure, and to characterise the gas content. We constrained the radial and vertical distribution of dust, as well as the presence of asymmetries. For a subset of six belts with CO gas, we constrained the gas distribution and kinematics. To interpret these observations, we used a wide range of dynamical models. The first ARKS results are presented as a series of ten papers. We discovered that up to 33% of our sample exhibits multiple dusty rings. For highly inclined belts, we found that non-Gaussian vertical distributions are common and are indicative of multiple dynamical populations. We also found that 10 of the 24 belts present asymmetries. We find that the CO gas is radially broader than the dust, but this could be an effect of optical depth. At least one system shows non-Keplerian kinematics due to strong pressure gradients, which may have triggered a vortex that trapped dust in an arc. Finally, we find evidence that the micron-sized grains may be affected by gas drag in gas rich systems. ARKS has revealed a great diversity of structures in exoKuiper belts that may arise when they are formed in protoplanetary discs or subsequently via interactions with planets and/or gas. We encourage the community to explore the reduced data and data products.

Figures (7)

• Figure 1: Distribution of fractional widths vs belt fluxes at 0.88 mm. Top: Histogram of all REASONS belts with resolved widths (grey) and those analysed in ARKS (blue, including six with archival observations). Middle: Distribution of fractional widths vs expected belt fluxes for moderately inclined belts. Bottom: Distribution of fractional widths vs expected belt fluxes for highly inclined belts. The blue markers represent ARKS targets (double markers: newly observed; single markers: archival data). The square symbols represent systems with CO gas, and the black crosses represent systems too large to be observed without mosaicking. The dashed line represents the 10 $\mu$Jy sensitivity limit chosen for ARKS (Eq. \ref{['eq:rms_fo']} and \ref{['eq:rms_eo']}), below which belts were excluded unless already observed (orange shaded region). The grey markers represent belts in REASONS that were excluded for being too large, too wide, or too faint for high-resolution observations.
• Figure 2: Distribution of stellar luminosities and ages for all systems in REASONS (grey) and in ARKS (blue). The symbols follow the same convention as in Fig. \ref{['fig:sample_selection']}. The top and right panels show histograms of the stellar age and luminosities, respectively.
• Figure 3: ARKS continuum clean images of the 24 systems in the sample after correction and subtraction of any SMG. The beam size is shown as a white ellipse in the bottom left corner. For sources imaged with a robust parameter greater than 0.5, an additional grey ellipse represents the beam size using a robust value of 0.5. The white ticks at the edges are spaced by 1, while the scale bar at the bottom right corner represents a projected distance of 50 au. The white cross represents the expected stellar position according to Gaia DR3, while the grey cross represents the best-fit centre of the system assuming a circular belt. For better clarity, each panel uses its own colour scale from $3\times$rms to the image peak.
• Figure 4: ARKS CO moment 0 images of the six systems with gas in the sample after subtracting the continuum ($^{12}$CO at the top and $^{13}$CO at the bottom row). We selected an appropriate robust parameter for each system to enhance the S/N per beam if necessary. The beam size is shown as a black ellipse in the bottom left corner. The black cross represents the expected stellar position according to Gaia DR3. The bar at the bottom of the top panels represents 50 au. The large and small ticks are spaced by 1 and 02, respectively. The imaging process is described in gas_arks. We note that HD 39060 does not have a $^{13}$CO image as it was not included in the archival observations that we use for ARKS.
• Figure 5: Same as Fig. \ref{['fig:gas_gallery_m0']}, but showing the moment 8 (peak intensity) images of the six systems with gas gas_arks.