exoALMA. XXI. The Morphology and Dynamics of Vertical Flows

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 the 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 bases of disk winds. In most cases, the velocity amplitudes are of a few tens of m s⁻¹. However, two disks, MWC758 and CQ Tau, show two spiral velocity features in their residual maps, red- and blueshifted, which we interpret as vertical velocities reaching up to ∼350 m s⁻¹ (∼0.7cₛ), consistent with the gas motion in eccentric disks. Fast upward motions (up to 500 m s⁻¹; ∼1.8cₛ) are 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 the identification of a coherent dominant physical mechanism driving the vertical motions across all disks, thus requiring further theoretical investigation.