The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) VIII: A dust arc and non-Keplerian gas kinematics in HD 121617

TL;DR

This study analyzes ALMA and SPHERE data of the gas-rich HD 121617 system from ARKS to characterize a millimetre dust arc and non-Keplerian CO gas kinematics. The authors model the dust as a two-component disc with a narrow azimuthally extended arc, finding the arc at $r_c\approx75$ au with an azimuthal FWHM of ~$90^{\circ}$ and contributing about $\sim13\%$ of the total dust mass, while small grains and gas show no strong arc. From gas kinematics, they extract a deprojected azimuthal velocity profile $v_\phi(r)$ that deviates from Keplerian due to a positive interior and negative exterior pressure gradient, enabling retrieval of the radial gas density profile under two mean-molecular-weight scenarios ($\mu=14$ or $\mu=2.3$). The density profile derived with $\mu\approx14$ matches the $^{13}$CO-based density, suggesting a secondary gas origin is plausible, though a primordial origin with sufficient shielding cannot be ruled out. The arc’s morphology and the non-Keplerian kinematics favor scenarios involving dust trapping in a vortex or planet–disc resonances, with future observations needed to distinguish these possibilities and to constrain the gas mass dynamically.

Abstract

ExoKuiper belts around young A-type stars often host CO gas, whose origin is still unclear. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) includes 6 of these gas-bearing belts, to characterise their dust and gas distributions and investigate the gas origin. As part of ARKS, we observed the gas-rich system HD121617 and discovered an arc of enhanced dust density. In this paper, we analyse in detail the dust and gas distributions and the gas kinematics of this system. We extracted radial and azimuthal profiles of the dust (in the millimetre and near-infrared) and gas emission ($^{12}$CO and $^{13}$CO) from reconstructed images. To constrain the morphology of the arc, we fitted an asymmetric model to the dust emission. To characterise the gas kinematics, we fitted a Keplerian model to the velocity map and extracted the azimuthal velocity profile by deprojecting the data. We find that the dust arc is narrow (1-5 au wide at a radius of 75 au), azimuthally extended, and asymmetric; the emission is more azimuthally compact in the direction of the system's rotation, and represents 13% of the total dust mass (0.2$M_\oplus$). The arc is much less pronounced or absent for small grains and gas. Finally, we find strong non-Keplerian azimuthal velocities at the inner and outer wings of the ring, as was expected due to strong pressure gradients. The dust arc resembles the asymmetries found in protoplanetary discs, often interpreted as the result of dust trapping in vortices. If the gas disc mass is high enough ($\gtrsim20M_\oplus$, requiring a primordial gas origin), both the radial confinement of the ring and the azimuthal arc may result from dust grains responding to gas drag. Alternatively, it could result from planet-disc interactions via mean motion resonances. Further studies should test these hypotheses and may provide a dynamical gas mass estimate in this CO-rich exoKuiper belt.

Figures (10)

• Figure 1: Dust and gas images of HD 121617. Left panel: Scattered light VLT/SPHERE $Q_\phi$ image at 1.25 $\mu$m ($J$ band) observed in polarised light with IRDIS scat_arks, smoothed with a Gaussian with a standard deviation of 1 pixel (12 mas). The central hatched region masks an area dominated by strong artefacts. The small orange circle in the SW ansa marks the pericentre location as constrained by scat_arks. Middle left panel: ALMA dust continuum image at 0.89 mm overview_arks. Middle right panel: $^{12}$CO J=3-2 moment 0 image gas_arks. Right panel: $^{13}$CO J=3-2 moment 0 image gas_arks. The white contours in the middle left image represent 75, 85, and 95% of the peak intensity. The 75% contour is also shown in the other images for reference. The ALMA continuum and CO images were obtained with CLEAN using robust parameters of 1.0 and 0.5, respectively. The beam and PSF sizes are shown as ellipses in the bottom left of each panel. The minor ticks in all panels are spaced by 02.
• Figure 2: Radial and azimuthal profiles extracted from the SPHERE and ALMA dust and gas images (produced with a robust parameter of 0.5). Top left panel: De-projected and azimuthally averaged intensity radial profiles extracted from wedges towards the NE and SW ansae with a width of 120° for ALMA and 60° for SPHERE. The bars on the top right indicate the resolution of each profile. Bottom left: Residual radial profiles after subtracting the average intensity of the NE from the SW ansa. Top right: Intensity azimuthal profiles obtained by averaging the emission between 65 and 90 au and over 40° rolling windows. Bottom right: residual azimuthal profiles after subtracting the mirrored profile relative to the NW minor axis of the disc (PA of 330°). The shaded coloured region in all panels represents the $1\sigma$ uncertainty taking into account the image noise and number of independent points being averaged (i.e. points spaced by a beam). The vertical shaded grey region in the left panels shows the radial region that was averaged when extracting the azimuthal profiles. The dust and gas intensities are normalised by their radial profile peaks.
• Figure 3: MCMC posterior distribution of the arc parameters used to fit the dust continuum observations: the peak amplitude of the arc $A_{\rm c}$; the central radius of the arc $r_{\rm c}$; the azimuthal location of the arc peak $\phi_{\rm c}$; the arc radial standard deviation $\sigma_r$; and the azimuthal standard deviations for the leading and trailing side of the arc $\sigma_{\phi,1}$ and $\sigma_{\phi,2}$, respectively. The contour levels in the 2D marginalised distributions correspond to the 68, 95 and 99.7% confidence levels. The dashed vertical lines in the marginalised distributions display the 16th, 50th and 84th percentiles. The red lines represent the best-fit value (lowest $\chi^{2}$). The posterior distribution of the other parameters is presented in Figure \ref{['fig:full_mcmc']}
• Figure 4: Comparison between observations and best-fit model. Top left: ALMA dust continuum image. Top right: Synthetic image of the best-fit model. Bottom left: Best-fit model image that has been beam-convolved. Bottom right: Dirty image of the residuals after subtracting the best-fit model from the observed visibilities. The dashed black and yellow contours represent positive and negative $3\sigma$ values, respectively. The white (grey) contours represent an intensity that is 30 and 75% the intensity peak of the observed (convolved model) image. The beam sizes are displayed in the bottom left corners. All images correspond to a CLEAN robust parameter of 1.0. The small white ticks are spaced by 0.2".
• Figure 5: Top panel: Line of sight $^{12}$CO velocity map. Bottom panel: Residual velocity map after subtracting a Keplerian model. The white-shaded region in the top panel represents pixels that did not have an S/N high enough to extract the line-of-sight velocity. The grey-shaded region in the bottom panel represents the area that was not included in the fit to avoid low-S/N areas. The white and grey contours display the location of the dust arc. The beam size is displayed in the bottom left corner.