The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) V: Comparison between scattered light and thermal emission

TL;DR

This work presents a coordinated comparison of dust distributions in exoKuiper belts by contrasting micron-sized grains seen in scattered light with millimetre-sized grains traced by ALMA for 24 ARKS targets. By applying uniform forward-modeling and non-parametric surface-density extractions, the study finds that about half of the discs detected in scattered light show no significant offset between the two tracers, while six gas-rich systems exhibit a notable outward displacement of the scattered-light peak relative to the ALMA peak, consistent with gas-dust interaction predictions. Known planets in several systems and SPHERE-based sensitivity analyses allow simultaneous constraints on unseen companions, with typical limits of 1–10 M$_\mathrm{Jup}$ beyond 10–20 au depending on age and distance. The results imply that gas presence can differentially affect small and large grains, pushing micron-sized dust outward in gas-rich discs, and motivate further hydrodynamic modeling and multi-wavelength follow-up (including JWST) to robustly interpret the observed substructures.

Abstract

Debris discs are analogues to our own Kuiper belt around main-sequence stars and are therefore referred to as exoKuiper belts. They have been resolved at high angular resolution at wavelengths spanning the optical to the submillimetre-millimetre regime. Short wavelengths probe the light scattered by such discs, which is dominated by micron-sized dust particles, while millimetre wavelengths probe the thermal emission of millimetre-sized particles. Determining differences in the dust distribution between millimetre- and micron-sized dust is fundamental to revealing the dynamical processes affecting the dust in debris discs. We aim to compare the scattered light from the discs of the ALMA survey to Resolve exoKuiper belt Substructures (ARKS) with the thermal emission probed by ALMA. We focus on the radial distribution of the dust. We used high-contrast scattered light observations obtained with VLT/SPHERE, GPI, and the HST to uniformly study the dust distribution in those systems and compare it to the dust distribution extracted from the ALMA observations carried out in the course of the ARKS project. We also set constraints on the presence of planets by using these high-contrast images combined with exoplanet evolutionary models. 15 of the 24 discs comprising the ARKS sample are detected in scattered light, with TYC9340-437-1 being imaged for the first time at near-infrared wavelengths. For 6 of those 15 discs, the dust surface density seen in scattered light peaks farther out compared to that observed with ALMA. These 6 discs except one are known to also host cold CO gas. Conversely, the systems without significant offsets are not known to host gas, except one. This observational study suggests that the presence of gas in debris discs may affect the small and large grains differently, pushing the small dust to greater distances where the gas is less abundant.

Figures (9)

• Figure 1: ARKS discs detected in scattered light. The observations were made using the VLT - SPHERE instrument, unless otherwise specified. The discs denoted by '$I$' were observed in total intensity, whereas the ones with '$pI$' were observed in polarised light. The white line at the bottom right corner represents an angular scale of 0.5, with the corresponding projected distance in au labelled in each panel. The white contours show the ALMA continuum observations, with three contour levels at a S/N of 3, 5 and 7 $\sigma$overview_arks. The white ellipse at the bottom left represents the ALMA image resolution and the white cross is the stellar position from GAIA DR3. For HD 131835, we show the image post-processed with the PACO algorithm Flasseur2018, best highlighting the two rings. In all panels, North is up, East to the left.
• Figure 2: Discs from the ARKS sample that have gas detections. The scattered light observations are the same as in Fig. \ref{['fig_mosaici']}. Overlaid on the scattered light images are the moment 0 maps for the gas observed with ALMA, using the robust values shown in gas_arks, i.e. 0.5 for $\beta$ Pictoris, HD 121617, 49 Ceti, HD 131835 and 2.0 for HD 131488 and HD 32297. The $^{12}$CO (J=3-2 line) is plotted in white and the $^{13}$CO (J=3-2 line) in green, with the exception of $\beta$ Pictoris where only the $^{12}$CO (J=2-1 line) is plotted, as this disc is not detected in $^{13}$CO. For every disc, there are 3 contour levels, corresponding to 3, 5, and 7 $\sigma$. The ellipses at the bottom left (white for $^{12}$CO and green for $^{13}$CO) represent the beam size for the ALMA observations. In all panels, North is up, East to the left.
• Figure 3: Comparison of the dust surface densities extracted from the ALMA continuum image and from the scattered light image in total intensity (I) or polarised intensity (pI). The scattered light profiles are rescaled to the ALMA profiles for display. For systems with CO detected, the $^{12}$CO or $^{13}$CO intensity profile is overplotted. Blue hatched regions correspond to inner regions where no scattered light modelling could be performed. The scale in the bottom right-hand corner in each panel indicates the ALMA and scattered light image resolution.
• Figure 4: Continued from Fig. \ref{['fig_radial_profile_comparison']}.
• Figure 5: SPHERE DPMs for the moderately inclined systems. The blue regions with contours at 99.7%, 95%, and 50% indicate the probability of detecting a planet at a 5$\sigma$ level with SPHERE. For three systems, HD 92945, HD 107146, and HD 206893, the 99.7% JWST/MIRI F1140C contour is shown in red for comparison. The orange hatched areas denote the 3R$_{\mathrm{Hill}}$ regions from the disc edges, where 1R$_{\mathrm{Hill}}$ is defined as Eq. \ref{['eq_hill_radius']}. Constraints from Gaia RUWE are indicated by grey dotted regions. The light blue curve marks the mass and location of a planet required to explain the significant proper motion anomaly. In systems with known planets, their positions are shown as orange dots.