The ALMA survey to Resolve exoKuiper belt Substructures (ARKS). X. Interpreting the peculiar dust rings around HD 131835

TL;DR

HD 131835 presents two rings at ~65 au and ~100 au with contrasting morphologies in scattered light and mm emission, plus CO gas interior to the inner ring. The authors test whether two independent planetesimal belts or a single belt with gas-driven dust migration can explain the data. Collisional-ACE modelling of two belts requires extreme differences in either dynamical excitation or material strength to match colours, while a gas-dust migration model can place an outer ring at ~100 au but underpredicts the outer mm-brightness, suggesting that a combined or more detailed treatment is needed. The results favor a plausible role for dust–gas interactions but indicate that fully self-consistent dynamical–collisional modelling and better gas constraints are needed to confirm the mechanism and fully reproduce all observational constraints. Overall, the work highlights the importance of gas in shaping debris-disc substructures and points toward JWST-era observations and advanced simulations to resolve the origin of HD 131835’s peculiar rings.

Abstract

Dusty discs detected around main-sequence stars are thought to be signs of planetesimal belts in which the dust distribution is shaped by collisional and dynamical processes, including interactions with gas if present. The debris disc around the young A-type star HD 131835 is composed of two dust rings at ~65 au and ~100 au, a third unconstrained innermost component, and a gaseous component centred at ~65 au. New ALMA observations show that the inner of the two dust rings is brighter than the outer one, in contrast with previous observations in scattered light. We explore two scenarios that could explain these observations: the two dust rings might represent distinct planetesimal belts with different collisional properties, or only the inner ring might contain planetesimals while the outer ring consists entirely of dust that has migrated outwards due to gas drag. To explore the first scenario, we employed a state-of-the-art collisional evolution code. To test the second scenario, we used a simple dynamical model of dust grain evolution in an optically thin gaseous disc. Collisional models of two planetesimal belts cannot fully reproduce the observations by only varying their dynamical excitation, and matching the data through a different material strength requires an extreme difference in dust composition. The gas-driven scenario can reproduce the location of the outer ring and the brightness ratio of the two rings from scattered light observations, but the resulting outer ring is too faint overall in both scattered light and sub-millimetre emission. The dust rings in HD 131835 could be produced from two planetesimal belts, although how these belts would attain the required extremely different properties needs to be explained. The dust-gas interaction is a plausible alternative explanation and deserves further study using a more comprehensive model.

Figures (12)

• Figure 1: HD 131835 observed in scattered light, using the VLT/SPHERE instrument. The white star at the centre denotes the position of the star. The white bar at the bottom right corner represents a distance of 0.5", with the corresponding value in au. In addition to the scattered light image, the contours of the corresponding ALMA dust continuum observations (thermal emission of bigger dust grains) are plotted. There are three contour levels, corresponding to [3, 5, 7] sigma. The white ellipse at the bottom left represents the beam size for the ALMA observation, using a robust value of 0.3. North is up and east is left.
• Figure 2: Normalized surface brightness of thermal emission from HD 131835 inferred from ALMA continuum (solid, dashed, and dotted red lines for the best-fit frank, rave, and parametric radial profiles, respectively) and of scattered light from VLT/SPHERE (blue line), and radial profile of intensity of molecular line emission of $^{12}$CO and $^{13}$CO observed with ALMA (solid and dashed orange lines; $^{13}$CO intensity is normalized in such a way that the $^{13}$CO/$^{12}$CO intensity ratio is preserved). The vertical dash-dot lines indicate the locations of the inner and the outer ring based on local maxima at 65.4 au and 96.5 au in the frank radial profile. The third, innermost component is partially within a region of high uncertainty for the scattered light observation (grey region) and appears to be asymmetric, and we do not include it in our theoretical models. For further details, see Section \ref{['sec:observations']}.
• Figure 3: Surface brightness profile of scattered light from the first modelling attempt (blue line). This is based on an assumption that the thermal emission observed with ALMA indicates the spatial distribution of planetesimals feeding an ideal collisional cascade, and the constraints on the SPHERE radial profile based on forward modelling (black line for the outer ring and upper limit for the inner ring). Vertical dash-dot lines indicate the locations of the inner and the outer ring based on local maxima at 65.4 au and 96.5 au in the frank radial profile. The inner 40 au are not included in the model as we focus on the outer regions. See Section \ref{['sec:challenge']} for details.
• Figure 4: Ratios of peak surface brightnesses at 890 µ m and 1.6 µ m (colours) shown as function of radii of peak thermal emission, for belt models with various initial excitations. Black dots represent reference cases with a maximum eccentricity $e_\text{max} = 0.1$, other symbols are as in the plot legend, with different sets of values explored in the two rings. The grey error bars show HD 131835 colours based on the ALMA and the SPHERE observations. Dashed and dotted lines give the expected surface brightness ratios for fiducial, unevolved discs with a fixed power-law size distribution and a minimum grain size of 1 µ m. For details, see Section \ref{['sec:ace_models']}.
• Figure 5: Sum of surface brightness profiles for some combinations of our inner and outer belt models, at 890 µ m (solid lines). In each combination the inner belt is the same ($e_{\rm max}=0.1$) and the outer belt has a different maximum eccentricity, as shown in the plot legend. Dotted lines only show the surface brightness of the outer belt models. The belts are modelled independently, and their peak surface brightness values are re-scaled to ring peak values from frank. The frank profile of HD 131835 is shown in red. For details, see Section \ref{['sec:ace_models']}.