The 12CO Gas Structures of Protoplanetary Disks in the Upper Scorpius Region

TL;DR

This study demonstrates that bulk gas-disk properties in protoplanetary disks can be robustly inferred from moderate-resolution ALMA 12CO J=3-2 visibilities by applying two parametric disk models within csalt. Analyzing 37 disks in the 4–14 Myr Upper Scorpius region, the authors derive gas-disk radii, CO emitting surfaces, and vertical height profiles, finding a median $R_{90\%}$ of about $84$ au, a positive brightness-temperature–stellar-luminosity correlation with $r\approx 0.6$, and a median emitting-layer aspect ratio $\langle z/r \rangle \approx 0.16$. They show that the $^{12}$CO optically thick layer sits below the super-heated dust layer, and they establish a significant $\langle z/r \rangle$–$R_{90\%}$ relationship, suggesting a fundamental coupling between vertical and radial disk structure across a range of disk masses and sizes. The work confirms the viability of extracting meaningful bulk disk properties from moderate-angular-resolution data and provides a practical framework for population studies of disk evolution in diverse star-forming regions.

Abstract

We present measurements of key protoplanetary disk properties inferred from parametric models of ALMA 12CO spectral line visibilities. We derive gas-disk radii, integrated fluxes, optically thick emission layers, and brightness temperature profiles for the disk population of the old (4 - 14 Myr) Upper Scorpius star-forming region. We measure CO emission sizes for 37 disks with bright CO J=3-2 emission (S/N > 10 on the integrated flux; out of the 83 disks with CO detections), finding that the median radius containing 90% of the flux is ~84 au, with radii spanning from 23 up to 243 au. We report a correlation between the 12CO brightness temperatures and stellar luminosities, with a Pearson coefficient of 0.6, and we use it to prove that the 12CO optically thick emission layer primarily emanates from a region below the super-heated dust, which is optically thin to the stellar irradiation. Moreover, we derive 33 CO emission surface height profiles, finding a median aspect ratio <z/r> ~ 0.16 in a range from ~0.01 up to ~0.45 over the sample. Finally, we comment on the multiple systems in our sample, of which only some were already known. These results re-affirm how it is possible to derive bulk disk properties by modeling moderate angular resolution ALMA visibilities.

Figures (21)

• Figure 1: Residual plot for J16213469-2612269. In the first row, we show the channel maps of the data; in the second row, we show the channel maps of the best-fit model; in the third row we show the channel maps of the residuals (data - model). The residual plots for all the disks analyzed in this work can be found in Appendix \ref{['appendix:residuals']}, and on GitHub\ref{['footnote:github']}.
• Figure 2: CCDF of the $^{12}$CO $R_{90\%}$ computed for this work (in blue) and of the radii evaluated using image-plane analysis from Zagaria et al., in prep. (in orange).
• Figure 3: Residual plot for J16062861-2121297, shown in the same way as in Fig. \ref{['fig:residuals_example']}. The residual plots for all the disks analyzed in this work can be found in Appendix \ref{['appendix:residuals']}, and on GitHub\ref{['footnote:github']}.
• Figure 4: Residual plot for J16123916-1859284, shown in the same way as in Fig. \ref{['fig:residuals_example']}. We highlight the residuals in the channel maps at $v=4.6, 5.5$ km s$^{-1}$, which are those affected by cloud absorption. Moreover, in the channel map at $v=3.7$ km s$^{-1}$ we show how csalt is able to separated the diffuse cloud emission (the localized red emission), and the keplerian disk emission. The residual plots for all the disks analyzed in this work can be found in Appendix \ref{['appendix:residuals']}, and on GitHub\ref{['footnote:github']}.
• Figure 5: CCDF of the $^{12}$CO $R_{90\%}$ for our sample (in blue) and of the sources in Upper Scorpius reported from Trapman_2025 (in orange). The gas-disk sizes reported in this figure are reported in Tables \ref{['Table:Fits_A']}, \ref{['Table:Fits_B']}.