Multi-frequency analysis of the ALMA and VLA high resolution continuum observations of the substructured disc around CI Tau. Preference for sub-mm-sized low-porosity amorphous carbon grains

TL;DR

This work uses high-resolution ALMA and VLA continuum data across 0.9 mm to 9.1 mm to jointly constrain dust temperature, density, and grain size in the CI Tau disc, explicitly testing how dust composition and porosity affect interpretive models. A two-step Bayesian radiative-transfer analysis that accounts for absorption and scattering shows that amorphous carbon grains with compact microphysics (Ricci, compact) best reproduce the observed surface-brightness and spectral-index profiles, yielding a near-constant maximum grain size of $a_{ m max}\approx (7.1\pm0.8)\times10^{-2}$ cm and marginal optical depths ($\tau_\nu \lesssim 1$) from 0.9 to 9.1 mm; porosity up to ~50% remains compatible, while graphite-rich mixtures are disfavored. The inferred dust density enhancements align with rings at ≈27, 60, and 152 au, and the grain-size distribution slope $q$ increases with radius, suggesting fragmentation- or bouncing-limited growth with ring confinement. The analysis also finds a substantial centimetre-wavelength non-dust contribution, likely including optically thick gyrosynchrotron emission, and estimates a modest pebble mass of ~31 $M_\oplus$ for the fiducial composition (with porosity allowing higher upper bounds up to ≈134 $M_\oplus$), implying that giant planet cores could be assembled in CI Tau under plausible efficiencies. These results demonstrate the power of combining multi-frequency, high-resolution data with physically constrained dust opacities to disentangle disc structure, dust evolution, and planet-formation potential, while highlighting the need for deeper cm-wavelength data and polarimetric observations to further tighten composition and porosity constraints.

Abstract

(Abridged) We present high angular resolution and sensitivity ALMA 3.1 mm and VLA 9.1 mm observations of the disc around CI Tau. These new data were combined with similar-resolution archival ALMA 0.9 and 1.3 mm observations and new and archival VLA 7.1 mm, 2.0, 3.0, and 6.0 cm photometry to study the properties of dust in this system. At wavelengths <3.1 mm, CI Tau's continuum emission is very extended and highly substructured (with three gaps, four rings, and two additional gap-ring pairs identified by non-parametric visibility modelling). Instead, the VLA 9.1 mm data are dominated by a bright central component, only partially (< 50%) due to dust emission, surrounded by a marginally detected, faint, and smooth halo. We fitted the ALMA and VLA 9.1 mm data together, adopting a physical model that accounts for the effects of dust absorption and scattering. For our fiducial dust composition ("Ricci" opacities), we retrieved a flat maximum grain size distribution across the disc radius of $(7.1\pm0.8)\times10^{-2}$ cm, that we tentatively attributed to fragmentation of fragile dust or bouncing. We tested, for the first time, the dependence of our results on the adopted dust composition model to assess which mixture can best reproduce the observations. We found that the "Ricci" opacities work better than the traditionally adopted "DSHARP" ones, while graphite-rich mixtures perform significantly worse. We also show that, for our fiducial composition, the data prefer low-porosity (< 70%) grains, in contrast with claims of highly porous aggregates in younger sources, which we tentatively justified by time-dependent compaction. Our results are in line with constraints from disc population synthesis models and naturally arise from CI Tau's peculiar spectral behaviour, making this disc an ideal target for deeper cm-wavelength and dust polarisation follow-ups.

Figures (26)

• Figure 1: From top to bottom: CI Tau's ALMA Band 7, 6, 3, and VLA Ka band continuum emission. Left column: CLEAN images. Central column: Azimuthally averaged surface brightness radial profiles. Those obtained from the tclean images are in violet, purple is used for the best-fit frank profiles (a point-source component was subtracted from the 3.1 and $9.1\,{\rm mm}$ visibilities before fitting). Right column: Residual images of the frank fit. Dotted ellipses mark the location of the dark rings in the CLEAN images. The synthesised CLEAN beam is shown as an ellipse in the bottom-left corner of each image and as a segment with full width half maximum equal to the beam minor axis in each radial profile subplot.
• Figure 2: CI Tau's spectral flux density distribution. The grey dots display CI Tau's photometry from this paper's data and those of Rodmann2006. Photometry by Chung2024 is over-plotted with black dots in the insert. Full markers show the total flux density (i.e. before point-source subtraction) for the ALMA Band 3 and the VLA Q and Ka band data. The maximum and minimum integrated flux densities across different scans are displayed as smaller dots for the Ku and X band data to highlight their short timescale variability.
• Figure 3: Spectral index radial profiles (solid lines) and their $1\sigma$ uncertainty (shaded areas). The hatched regions mark those locations where ${\rm S/N}\leq5$ (left and central panel) and 3 (right panel), for at least one of the emission profiles. The dashed grey lines in each panel display the surface brightness radial profiles combined to determine the spectral index.
• Figure 4: Dust temperature, density, maximum size, and density distribution power-law index posterior distributions for our fiducial 'Ricci (compact)' composition. The blue solid lines and shaded regions display the median and $1\sigma$ uncertainty of each parameter. For comparison, the dotted lines show the results of the low resolution fit. The hatched areas mark the disc region within the synthesised beam minor axis ($0\farcs058$) and, in the outer disc, the region where our fit does not provide robust results due to the low S/N of the data. The grey dashed line displays our ALMA Band 6 surface brightness radial profile (at a resolution of $0\farcs058$), the bright ring position is indicated and labelled as in the previous plots. The black and grey horizontal arrows mark the regions where the S/N of VLA Ka band surface brightness radial profile is $>5$ and 3.
• Figure 5: Optical depth, absorption optical depth, albedo, and total dust opacity posterior distributions at 0.9, 1.3, 3.1, and $9.1\,{\rm mm}$ for our fiducial 'Ricci (compact)' composition. The solid lines and shaded regions display the median and $1\sigma$ uncertainty of each parameter. For comparison, the dotted lines show the results of the low resolution fit. The black dotted lines display the absorption optical depth estimated at each wavelength separately in the optically thin approximation from \ref{['eq:5.1']}. Our ALMA Band 6 surface brightness radial profile is plotted with grey dashed lines. , the bright ring position is indicated and labelled as in the previous plots. The black and grey horizontal arrows mark the regions where the S/N of VLA Ka band surface brightness radial profile is $>5$ and 3. Dust emission is marginally optically thin at all the wavelengths.