FAUST. XXVIII. High-Resolution ALMA Observations of Class 0/I Disks: Structure, Optical Depths, and Temperatures

TL;DR

This work uses ~7.5 au resolution ALMA observations at 1.3 and 3 mm to study 16 Class 0/I protostellar disks and 3 circumbinary disks, revealing extensive optically thick emission that drives high disk luminosities and challenges simple optically thin mass estimates. Through matched-resolution imaging, spectral index maps, and profile fits, it shows that most disks are optically thick with $\alpha\approx2$ but with edges where $\alpha$ rises above 2 and brightness temperatures exceed irradiation-based expectations, implying additional heating such as viscous processes. Disk sizes at 1.3 and 3 mm are nearly identical, with larger disks systematically brighter by about an order of magnitude than Class II counterparts, consistent with higher temperatures and optical depth fractions. The study further finds substantial disk masses (0.01–0.3 $M_\odot$ in gas) and several disks near the Toomre $Q\sim1-2$ regime, suggesting possible marginal gravitational instability in the early stages, and identifies CBDs with relatively low optical depths and significant dust masses, indicating ongoing grain growth and potential planet-forming conditions early on. Overall, the results support early planet formation scenarios under high optical depth and highlight the need for longer-wavelength data to robustly constrain grain growth and kinematics in these young disks.

Abstract

We present high-resolution (~7.5 au) ALMA observations at 1.3 and 3 mm of 16 disks around Class 0/I protostars across multiple star-forming regions and a variety of multiplicities, showing a range of disk sizes (~2-100 au) and including circumbinary disks (CBDs) in binaries with separations <100 au. The disk properties show similarities to Class II disks, including (a) low spectral index (SI) values (alpha=2.1) that increase with disk radius, (b) 3 mm disk sizes only marginally smaller than at 1.3 mm (<10%), and (c) radial intensity profiles well described by modified self-similar profiles. We also find key differences: (i) SI values increasing with radius, but exceeding 2 only at the disk edge (ii) higher brightness temperatures Tb, in some cases higher than the predicted temperatures due to irradiation, and (iii) ~10x higher luminosity at a given size compared to the Class II disks. These results confirm significant optical depth in the observed Class 0/I disks, at both 1.3 and 3 mm, helping to explain their higher luminosities, but higher temperatures are also required for the most compact (< 40 au) disks, suggesting additional viscous heating. Considering optical depth, most disk dust masses are estimated in the range 30-900 Mearth (0.01-0.3 Msun in gas), resulting in some disks reaching marginal gravitational instability. The median location of the water iceline is ~3 au, but it can extend beyond 10-20 au for the hottest disks. CBDs exhibit lower optical depths at both wavelengths and hence higher SI values (alpha=3.0), dust masses of 100 Mearth, and beta~1.5 (2 Class 0 CBDs) and beta~1 (1 Class I CBD), suggesting substantial grain growth only in the more evolved CBD. The inferred high optical depths provide a compelling explanation for the apparent scarcity of dust substructures in the younger disks at ~ 1 mm, despite mounting evidence for early planet formation (ABRIDGED).

Figures (24)

• Figure 1: ALMA 3 mm images of all the sources in our sample at the same physical scale. The emission is presented with a log stretch to enhance the weaker extended emission. The images are organized in two groups. The first one (top three rows) correspond to all the systems in which the projected separation to the nearest protostellar neighbor is larger than 100 au, while the second one (bottom row) comprises the systems with a protostellar neighbor below 100 au. Unlike the first group, disk-like circumbinary structures are observed for all sources in the second group. For each group the sources are organized by increasing bolometric temperature.
• Figure 2: Same as Figure \ref{['fig:obs_3mm']}, for the 1.3 mm observations. Some sources detected at 3 mm and not observed at 1.3 mm due to the smaller field of view.
• Figure 3: Maps of 1.3-3 mm spectral index for all sources with observations at 1.3 and 3 mm. The ordering is the same as in Figures \ref{['fig:obs_3mm']} and \ref{['fig:obs_1mm']}. The upper corner of each panel shows the minimum and maximum value within the map, considering only pixels with errors below 0.2 (including both statistical and flux calibration uncertainties).
• Figure 4: Spectral index distribution of all disks versus circumbinary material, obtained through a KDE. The vertical lines at the bottom show the mean value of the spectral index for the individual disk sources and circumbinary structures
• Figure 5: Intensity profiles on logarithmic scales at 1.3 and 3 mm expressed in brightness temperature, calculated using the full Planck function. The shaded area corresponds to the uncertainty in the mean (Section \ref{['sec:intensity_profiles']}). The 1.3-3 mm spectral index $\alpha$ calculated from the profiles is shown in magenta with the corresponding value in the right y-axis. The magenta shaded area shows the uncertainty calculated propagating the errors in the intensity profiles. The vertical dotted line represents the geometric mean of the beam size, indicating that emission beyond this point is spatially well resolved.