2025A&A...702A..56Z

We present high angular resolution (50 mas) and sensitivity Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 (3.1 mm) and Very Large Array (VLA) Ka band (9.1 mm) observations of the multi-ringed disc around the 3 Myr-old solar-mass star CI Tau. These new data were combined with similar-resolution archival ALMA Band 7 (0.9 mm) and 6 (1.3 mm) observations and new and archival VLA Q (7.1 mm), Ku (2.0 cm), X (3.0 cm), and C band (6.0 cm) photometry to study the properties of dust in this system. At wavelengths ≤3.1 mm, the continuum emission from CI Tau is very extended (≥200 au) and highly substructured (with three gaps, four rings, and two additional gap-ring pairs identified by non-parametric visibility modelling). In contrast, the VLA Ka band data are dominated by a centrally peaked bright component, only partially (≤50%) due to dust emission, surrounded by a marginally detected faint and smooth halo. We fitted the ALMA and VLA Ka band 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, with a_max = (7.1 ± 0.8) × 10^–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 '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. This is in contrast with recent claims of highly porous aggregates in younger sources, which we tentatively justified by time-dependent compaction at the fragmentation or bouncing barrier. Our results on composition and porosity are in line with constraints from disc population synthesis models and naturally arise from CI Tau's peculiar spectral behaviour (i.e. the abrupt steepening of its spectral index at wavelengths longer than 3.1 mm), making this disc a unique target to characterise the properties of disc solids and thus ideal for deeper centimetre-wavelength observations and follow-up dust polarisation studies.