The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) IX: Gas-driven origin for the continuum arc in the debris disc of HD 121617

TL;DR

This work demonstrates that gas–dust hydrodynamics, including radiation pressure and dust feedback, can generate the azimuthal continuum arc observed in HD 121617 by trapping dust in a marginally unstable gas ring. By performing 2D Dusty FARGO-ADSG simulations and radiative-transfer image synthesis, the authors constrain the total gas mass to between roughly $2.5$ and $250\,M_\oplus$, with viable cases at $50$ and $5\,M_\oplus$ that reproduce both the ALMA continuum arc and the SPHERE scattered-light offset. The results imply a hybrid-disc scenario in which primordial gas persists alongside debris-dominated dust, and they propose multi-band continuum imaging and time-domain monitoring as decisive tests to distinguish gas-drag trapping from planet-induced resonances, thereby offering a pathway to quantify hidden gas reservoirs in debris discs and illuminate late-stage disc evolution.

Abstract

Debris discs were long considered to be largely gas-free environments governed by collisional fragmentation, gravitational stirring, and radiative forces. Recent CO detections show that gas is present, but its abundance and origin remain uncertain. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) revealed a narrow gas and dust ring in the disc HD 121617 with an asymmetric arc 40% brighter than the rest of the ring. We aim to constrain the total gas mass in HD 121617 assuming the dust arc is produced by hydrodynamical gas-dust interactions. We used the Dusty FARGO-ADSG code, modelling dust as Lagrangian particles, including radiation pressure and dust feedback, and varying the total gas mass. Simulations were compared to observations using radiative transfer. An unstable gas ring creates a size-dependent radial and azimuthal dust trap whose efficiency depends on gas mass. Two models, with 50 and 5 Earth masses of gas, reproduce both the ALMA band 7 arc and the outward offset of the VLT/SPHERE scattered-light ring via gas drag and radiation pressure. We infer a conservative gas-mass range of 2.5 to 250 Earth masses. If the ALMA asymmetry is caused by gas drag, the required gas mass compared with the observed CO implies substantial H2, consistent with primordial gas. HD 121617 would then be a hybrid disc between protoplanetary and debris stages. Since a planet could also create an arc, future observations are needed to distinguish these scenarios.

Figures (20)

• Figure 1: Observations of HD 121617. The left panel shows the ALMA band 7 continuum observation ($\lambda_{\rm obs}=0.89\,$mm) from overview_arks. The contours at $[80,160,200]\,\mu{\rm Jy}/{\rm beam}$ highlight the ring's azimuthal asymmetry. The clean beam is displayed as a white ellipse in the bottom left. The right panel shows the SPHERE/IRDIS $J$-band scattered light image ($\lambda_{\rm obs}=1.25\,\mu$m) from scat_arks, with the same ALMA continuum contours. The grey circle indicates the coronagraphic mask of the observation.
• Figure 2: One-dimensional simulations of a dust ring. All setups included radiation pressure with $\beta_{1\mu{\rm m}} \!=\! 0.4$, and are shown after 150 orbits at $r_0$. Left: Simulation without gas. The horizontal line indicates the minimum particle size for which the dust remains on bound orbits, and the shaded area marks the initial radial range of the dust particles. Centre: Includes gas density specified by Eq. (\ref{['equ:dens0']}), with gas density ($\Sigma_\mathrm{g}$, blue curve) normalised to its maximum value, which was set to represent a gas ring of $10\,M_\oplus$, representing a primordial scenario. The dust-to-gas ratio is 0.01. The dashed curve shows where $\beta \!=\! \eta$. Right: Same as the centre panel, but with $M_{\rm gas}\!=\!0.1\,M_\oplus$, and a dust-to-gas ratio of unity, reflecting a scenario of secondary gas origin. Large particles are affected by neither radiation pressure nor gas drag, and they remain on their initial circular Keplerian orbits. For sizes $\lesssim\!100\,\mu\mathrm{m}$, both gas drag and radiation pressure become efficient, and the particles move outward. The momentum feedback from the dust changes the gas density profile from its initial distribution (transparent blue line).
• Figure 3: Fiducial hydrodynamical model after 150 orbits at $r_0$ (corresponding to about 71 kyr). The total gas mass of the Gaussian ring is $M_{\rm g} \!=\! 50\,M_\oplus$, with a dust-to-gas mass ratio of $4.2\!\times\!10^{-3}$. Left: Gas surface density distribution. Right: Particle distribution with colour indicating the grain size.
• Figure 4: Radial distribution of particles corresponding to Fig. \ref{['fig:hydro_fiducial']}. The figure incorporates every tenth particle. The black curve traces the azimuthally-averaged gas surface density profile. The vertical dashed line marks the pressure maximum. The purple curve indicates the particle size that corresponds to a Stokes number of unity.
• Figure 5: Azimuthal distribution of particles of different sizes corresponding to Fig. \ref{['fig:hydro_fiducial']}. The figure incorporates every tenth particle. The black curve traces the radial peak value of the gas surface density and corresponds to the right y-axis.