FAUST

XXVIII. High-resolution ALMA observations of Class 0/I disks: Structure, optical depths, and temperatures

¹ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstr. 1, 85741 Garching, Germany
² Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC; Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei 116, Taiwan, ROC
³ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi”, Viale Berti Pichat 6/2, Bologna, Italy
⁵ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁷ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
⁸ Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
⁹ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia 58089, Mexico
¹⁰ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
¹¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² European Southern Observatory, Karl-Schwarzschild-Str 2, 85748 Garching, Germany
¹³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Region Metropolitana de Santiago, Chile
¹⁴ Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto-shi, Kyoto-fu 606-8502, Japan
¹⁵ Institut de Radioastronomie Millimétrique, 38406 Saint-Martin d’Héres, France
¹⁶ RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
¹⁷ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitakashi, Tokyo 181-8588, Japan
¹⁸ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁹ Department of Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang, Shanghai 200240, PR China
²⁰ SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, Kanagawa 240-0193, Japan
²¹ David Rockefeller Center for Latin American Studies, Harvard University, 1730 Cambridge Street, Cambridge, MA 02138, USA

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Received: 23 June 2025
Accepted: 21 October 2025

Abstract

Measuring the properties of disks around Class 0/I protostars is crucial for understanding protostellar assembly and early planet formation. We present high-resolution (~7.5 au) ALMA continuum observations at 1.3 and 3 mm of 16 disks around Class 0/I protostars across multiple star-forming regions (Taurus, Ophiuchus, and Corona Australis) and a variety of multiplicities. Our observations show a wide range of deconvolved disk sizes (~2–100 au) and the presence of circumbinary disks (CBDs) in all binaries with separations <100 au. The measured properties show similarities to Class II disks, including (a) low spectral index values (α_disks = 2.1_−0.3^+0.5) that increase with disk radius, (b) 3 mm disk sizes only marginally smaller than at 1.3 mm (<10%), and (c) radial intensity morphologies well described by modified self-similar profiles. However, there are some key differences: (i) the α_{1.3-3 mm} values increase monotonically with radius but exceed two only at the disk edge; (ii) higher brightness temperatures, T_b, comparable to or higher than the predicted midplane temperatures due to irradiation; and (iii) an approximately ten times higher luminosity at a given size compared to the Class II disks. Together, the results confirm significant optical depth in the observed Class 0/I disks, most with T_bol < 200 K, at both 1.3 and 3 mm. Assuming fully optically thick disks at these wavelengths can explain the higher luminosities compared with Class II disks, but the most compact (≲40 au) disks also require higher temperatures, suggesting additional heating from viscous accretion. Taking into account the high optical depths, most disk dust masses are estimated in the range 30–900 M_⊕ (or 0.01–0.3 M_⊙ in gas), with some disks potentially reaching marginal gravitational instability. Based on the elevated T_b^1.3 mm, the median location of the water iceline is ~3 au, but this location can extend to more than 10–20 au for the hottest disks in the sample. The CBDs exhibit lower optical depths at both wavelengths and hence higher spectral index values (τ_{3 mm} ≲ 1, α_CBD = 3.0_−0.3^+0.2), dust masses of ~10² M_⊕, and dust emissivity indices of β_CBD ~ 1.5 (two Class 0 CBDs) and ~1 (one Class I CBD), suggesting substantial grain growth only in the more evolved CBD. The high optical depths inferred from our analysis provide a compelling explanation for the apparent scarcity of dust substructures in the younger Class 0/I disks at ~1 mm despite the mounting evidence of early planet formation.