exoALMA XXI: The Morphology and Dynamics of Vertical Flows

Abstract

Vertical gas flows, such as winds and meridional circulations, are natural outcomes of protoplanetary disk processes and play a critical role in the earliest stages of planet formation. We analyze vertical gas motions in 14 disks as part of the exoALMA Large Program, focusing on the 12CO J=3-2 and 13CO J=3-2 emission lines. Using discminer to model the Keplerian velocity field, we extract line-of-sight velocity residuals and measure the radial and vertical components of the gas motion. Vertical motions are detected in most disks. Two types of patterns emerge: (1) oscillatory up/down flows, likely linked to instabilities, and (2) transitions from downward to upward motions that we interpret as the base of a disk wind. In most cases, the velocity amplitudes are of a few tens of m/s. Two disks, however, MWC758 and CQ Tau, show two spiral velocity features in their residual maps, red- and blue-shifted, which we interpret as vertical velocities reaching up to 350 m/s (0.7 Cs), consistent with gas motion in eccentric disks. Fast upward motions (up to 500 m/s; 1.8 Cs) is also detected in the outer disk of MWC758. Synthetic observations from (magneto)hydrodynamic simulations validate the reliability of our method. Although strong molecular winds appear to be relatively rare in 12CO and 13CO, our study shows that, when traced by deep high spectral resolution line data, protoplanetary disks exhibit ubiquitous vertical flows. However, their overall velocity structure is highly complex, preventing to identify a coherent, dominant physical mechanism driving the vertical motions across all disks, thus requiring further theoretical investigation.

Figures (15)

• Figure 1: Sketch explaining the types of velocity residuals expected after subtraction of a Keplerian disk model, in 2D residual maps (labeled 'residuals') and in folded residual maps (labeled 'folded'). A more comprehensive sketch encompassing more cases can be found in exoALMA XX/Izquierdo et al. subm.
• Figure 2: $^{12}$CO line of sight residual maps for exoALMA targets. Note the different range of velocity amplitudes.
• Figure 3: Benchmark of our $\upsilon_{\rm z}$ retrieval method with hydrodynamical models. In the top, we show folded residual maps from MRI, VSI, GI and planet-disk interaction models. For each subset of two panels, we present $\upsilon_{\rm z}$ from the model and the ones retrieved with discminer (top), as well as the standard deviation of the folded map and the difference between the model and measured $\upsilon_{\rm z}$ (bottom), all against radius.
• Figure 4: Top: maximum upward/downward $^{12}$CO vertical velocities for the exoALMA targets for which we apply a 1D analysis. On the right, the 3 MAPS targets. Bottom: differences in $\upsilon_{\rm sys}$ computed from the $^{12}$CO line and CS (for exoALMA), and $^{12}$CO and C$^{18}$O (for MAPS).
• Figure 5: Gallery of azimuthally averaged vertical (top panels) and radial velocity (bottom panels) profiles. Blue lines are $^{12}$CO and yellow lines are $^{13}$CO. The errors are the standard deviation along each annulus. On the right, the folded residual maps for $^{12}$CO are presented, in which any significant coherent vertical flow appears as an arc of uniform color (blue or red).