2026A&A...705A..96M

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.