exoALMA XIX: Confirmation of Non-thermal Line Broadening in the DM Tau Protoplanetary Disk

TL;DR

This study combines high-resolution exoALMA observations of DM Tau with full radiative transfer modeling (MCForSt) and Bayesian inference to quantify non-thermal line broadening in a protoplanetary disk. The CO J=3-2 fit yields a robust turbulence level of $f_{\rm turb} \approx 0.40$ relative to the local sound speed, implying $\alpha$ of order $0.16$ and non-thermal velocities around $180$ m s$^{-1}$, inconsistent with purely thermal motions. The CS J=7-6 emission is well reproduced using the CO-based disk structure with a nearly constant CS abundance and minimal freeze-out, suggesting non-equilibrium chemistry and photodesorption sustain gas-phase CS in the cold disk. Substructure in the residual maps points to localized perturbations that could trace forming planets. Overall, the approach provides a powerful pathway to extract disk structure, turbulence, and chemistry directly from line data and can be applied to other high-quality disks.

Abstract

Turbulence is expected to transport angular momentum and drive mass accretion in protoplanetary disks. One way to directly measure turbulent motion in disks is through molecular line broadening. DM Tau is one of only a few disks with claimed detection of nonthermal line broadening of 0.25cs-0.33cs, where cs is the sound speed. Using the radiative transfer code mcfost within a Bayesian inference framework that evaluates over five million disk models to efficiently sample the parameter space, we fit high-resolution (0.15", 28 m s-1) 12CO J = 3-2 observations of DM Tau from the exoALMA Large Program. This approach enables us to simultaneously constrain the disk structure and kinematics, revealing a significant nonthermal contribution to the line width of ~0.4cs, inconsistent with purely thermal motions. Using the CO-based disk structure as a starting point, we reproduce the CS J = 7-6 emission well, demonstrating that the CS (which is more sensitive to nonthermal motions than CO) agrees with the turbulence inferred from the CO fit. Establishing a well-constrained background disk model further allows us to identify residual structures in the moment maps that deviate from the expected emission, revealing localized perturbations that may trace forming planets. This framework provides a powerful general approach for extracting disk structure and nonthermal broadening directly from molecular line data and can be applied to other disks with high-quality observations.

Figures (12)

• Figure 1: Channel maps of DM Tau in $^{12}$CO $J$=3–2 at 015 resolution. The top row shows the observed data, the second row our best-fit model with non-zero turbulence, and the third row the residuals from subtracting this model from the data. The fourth and fifth rows show the model and residuals for the best fit zero-turbulence model for comparison. Each residual color bar spans $\pm3\sigma$ flux for the data cube.
• Figure 2: Comparison of observed and modeled integrated line profiles for DM Tau. Left: CO $J$=3–2 line profile. Right: CS $J$=7–6 line profile. The vertical dotted lines on each panel mark the velocity channels included in the fit. The agreement for CS demonstrates that the CO-derived disk structure can be directly applied to other molecular tracers, allowing differences in emission to be interpreted primarily in terms of chemistry and excitation rather than large-scale structural variations.
• Figure 3: CO number density in DM Tau. The cyan and magenta dashed lines show the exoALMA-derived emission heights from Galloway-Sprietsma_exoALMA using DiscMiner and disksurf. Left: $J=2-1$ model from flaherty20. Right: Our $J=3-2$ model.
• Figure 4: Comparison of temperature structures from different models. Each panel shows the gas temperature as a function of radius and height for (left to right) the models from flaherty20, Galloway-Sprietsma_exoALMA, and this work. The white contour indicates 20 K, which we used as our CO freeze-out temperature. The black contours trace the height enclosing the top 95% of the model CO number density at each radius, highlighting the bulk CO while excluding low-density photodesorbed regions. Colored contours indicate the exoALMA emission heights.
• Figure 5: Channel maps of DM Tau in CS J=7--6 emission at 015 resolution. The top row shows the observed data, the middle row the equivalent channels for our best-fit model and the bottom row the residuals from subtracting this model from the data. The residual color bar represents $\pm3\sigma$ flux of the data.