The ALMA survey to Resolve exoKuiper belt Substructures (ARKS): VI. Asymmetries and offsets

TL;DR

ARKS VI addresses how common and how varied continuum asymmetries and stellocentric offsets are in nearby debris discs observed with ALMA. Using empirical, image-domain techniques (2D self-subtractions and 1D azimuthal/axis profiles) and Gaia-based astrometry, the study identifies ten systems with significant asymmetries or offsets, including three eccentric discs (HD 15115, HD 32297, HD 109573) and multiple azimuthal arcs. The results reveal a broad, yet subtle, landscape of non-axisymmetric features that correlate with the cold dust fractional luminosity and show tentative ties to CO-rich gas, hinting at gas–dust interactions as a partial driver. The findings corroborate that axisymmetric models capture bulk disc properties, while non-axisymmetric features are common and merit further high-resolution, multi-wavelength follow-up and dynamical modelling to uncover underlying planetary or dynamical causes.

Abstract

Asymmetries in debris discs provide unique clues to understand the evolution and architecture of planetary systems.** The aim of the ALMA survey to Resolve exoKuiper belt Substructures (ARKS) is to expand our understanding of radial and vertical dust structures, as well as gas distributions and kinematics, in debris discs.** Here, in ARKS~VI, we present a systematic analysis of the asymmetries and stellocentric offsets present in the ALMA continuum data for the ARKS survey. Our aims are to identify asymmetries in debris disc dust distributions, quantify debris disc asymmetry properties, and discuss the potential origins of debris disc asymmetries.** We utilised empirical methods to identify emission asymmetries** and the presence of offset emission between disc centres and the locations of the host stars, via an analysis of their calibration procedures and disc properties. We associated observational asymmetry types** and plausible physical classes** associated with each source. We show that there are ten systems, almost half of the ARKS sample, that host either a continuum emission asymmetry or offset emission. Three systems host offsets (HD15115, HD32297, and HD109573 (HR4796)), four host azimuthal asymmetries (HD9672 (49Ceti), HD92945, HD107146, and HD121617), two host an asymmetry in their major axis (HD10647 (q$^1$ Eri), and HD39060 ($β$ Pic)), and one hosts an asymmetry in their minor axis (HD61005). We attribute the offset asymmetries to non-zero eccentricities, and three of the azimuthal asymmetries to arcs. The presence of an asymmetry or offset in the ARKS sample appears to be correlated with the fractional luminosity of cold dust.** Conclusions: This study demonstrates that debris disc asymmetries in the ARKS sample are common, and plausibly so in the wider population of debris discs at (sub)-millimetre wavelengths.** ** = ABRIDGED FOR ARXIV: FULL ABSTRACT IN PAPER

Figures (13)

• Figure 1: Gallery of the ten ARKS systems with evidence of asymmetric or offset emission, and their respective observational asymmetry type. The arrows indicate systems that host a major or minor axis with the directionality pointing towards the brighter side. All systems are shown with different contour levels to accentuate their asymmetric features. We present the synthesised beams in the lower left of each panel, as well as 50 au scale bars in the lower right. For systems with significant offsets, we show insets zoomed into their central regions, denoting their phase (stellar) centres as orange stars and best-fit disc centres as blue dots. In all panels north is up and east is left. From left to right, then top to bottom, the images are cropped to fields of view that in both axes are ${\pm}7.5"$, ${\pm}8"$, ${\pm}0.875"$, ${\pm}2.5"$, ${\pm}1"$, ${\pm}1.125"$, ${\pm}9"$, ${\pm}8"$, ${\pm}3"$, and ${\pm}4"$, respectively.
• Figure 2: Top: HD 9672 residual map as presented in ARKS III, ver_arks, with the ARKS continuum image shown in solid black contours, and the $^{12}$CO (3-2) moment-zero residuals overplotted as thick dash-dot green contours, as presented in Hughes2017, cropped to a $\pm4"$ region of the sky. The continuum disc position angle is shown as the black dotted line across the disc. Clean beams, contour scales, and a scale bar are shown on the map. North is up; east is left. Bottom: Minor axis residual intensity profile across a major axis averaged region about the ansae regions, shown in solid red (after reversing the direction of the western ansae). Errors are shown at the ${\pm}1\sigma$ level, scaled to account for the mean and difference profiles.
• Figure 3: HD 10647 major axis profiles (top) for each of the archival-only, ARKS-only, or combined ('All') baselines, and their associated difference profiles (bottom) obtained by self-subtracting the profiles about $0.0"$. The major axis direction is negative towards the south-east of the disc. The red and green dash-dot lines represent the ${\pm}3\sigma$ and ${\pm}5\sigma$ profile differences for the combined data; the shading gives the error associated with each of the difference profiles, scaled by $\sqrt{2}$ to account for the self-subtraction. We also show the ${\approx}40{-}80\,$au radial location (in blue) associated with q$^1$ Eri's dust clump from Lovell21c.
• Figure 4: HD 39060 ($\beta$ Pic) major axis profile (top) from the south (minus major axis direction) to the north (positive radial direction), and its associated major axis difference profile about the image centre (bottom). We show in red and green dash-dot lines ${\pm}3\sigma$ and ${\pm}5\sigma$ profile differences, and in amber the error associated with the difference profile, scaled by $\sqrt{2}$ to account for the uncertainty in the self-subtraction of the profile. We also show the ${\approx}85\,$au radial location (in blue) associated with $\beta$ Pic's known CO clump from Dent14.
• Figure 5: HD 61005 minor axis profile (top) from the south (minus minor axis direction) to the north (positive minor axis direction), and its associated difference profile about the image centre (bottom). We show as red and green dash-dot lines the $\pm 3\sigma$ and $\pm 5\sigma$ profile differences, and in amber the error associated with the difference profile, scaled by ${\sqrt{2}}$ to account for the uncertainty in the self-subtraction of the profile.