Multi-frequency analysis of the ALMA and VLA high resolution continuum observations of the substructured disc around CI Tau

Preference for submillimetre-sized low-porosity amorphous carbon grains

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
³ Università degli Studi di Milano, Via Giovanni Celoria 16, 20133 Milano, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁵ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁶ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
⁷ INAF – Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
⁸ Astrophysics Group, Department of Physics, Imperial College London, Prince Consort Rd, London SW7 2A2, UK
⁹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
¹² Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40122 Bologna, Italy

^★ Corresponding author: frzagaria@mpia.de

Received: 13 November 2024
Accepted: 10 July 2025

Abstract

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.