exoALMA. XXI. The Morphology and Dynamics of Vertical Flows

Overview

This study investigates vertical gas flows in 14 protoplanetary disks observed as part of the exoALMA Large Program, using high-resolution molecular line observations to characterize wind and circulation patterns in the earliest stages of planet formation. Analysis of 12CO and 13CO J=3-2 emission lines reveals diverse vertical velocity signatures ranging from tens to hundreds of meters per second, indicating that ubiquitous vertical motions characterize these systems despite heterogeneous physical origins.

Methods and approach

Observations of 14 disks were conducted using carbon monoxide isotopologue transitions (12CO J=3-2 and 13CO J=3-2). The discminer modeling tool was applied to isolate residual line-of-sight velocities from the Keplerian velocity field, enabling extraction of radial and vertical gas motion components. Velocity amplitudes and spatial patterns were measured across the disk sample. Synthetic observations generated from magnetohydrodynamic and hydrodynamic simulations were used to validate the observational methodology and interpretation reliability.

Key Findings

Vertical motions were detected in most disks studied. Two primary flow patterns emerged: oscillatory up-and-down flows interpreted as instability-driven processes, and transitions from downward to upward motions consistent with disk wind bases. Typical velocity amplitudes reach tens of meters per second. Two disks, MWC758 and CQ Tau, exhibited spiral velocity features with red- and blueshifted components reaching approximately 350 m/s (0.7 sound speed), consistent with gas dynamics in eccentric disk structures. MWC758 additionally displayed fast upward motions up to 500 m/s (1.8 sound speed) in the outer disk region.

Implications

The detection of ubiquitous vertical flows across the disk sample, when traced by deep high-spectral-resolution molecular line data, establishes vertical gas motion as a fundamental characteristic of protoplanetary disk structure. However, the heterogeneity of velocity patterns and amplitudes indicates that no single dominant physical mechanism uniformly drives vertical motions across diverse disk systems, necessitating expanded theoretical investigation into the multiple processes governing vertical gas transport. Strong molecular winds traced in CO transitions appear relatively uncommon, suggesting that wind signatures may be localized or traced more effectively by alternative molecular species or observational approaches.