ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT) VII: the layered molecular outflow from HL Tau and its relationship with the ringed disk

TL;DR

This study uses ALMA observations to analyze the layered molecular outflow from HL Tau, revealing nested shells and their connection to disk substructures, supporting models of magnetic winds and disk evolution.

Contribution

It provides detailed characterization of the outflow's layered structure and links wind launch points to disk rings, offering insights into disk wind mechanisms and planet formation.

Findings

Nested shells with increasing velocity and size are observed in the outflow.

Wind launch radii correspond to specific disk rings at 58, 72, and 86 au.

The magnetic lever arm is estimated to be 4-5, indicating efficient angular momentum removal.

Abstract

The ringed disk around HL Tau stands out as the iconic signature of planet formation, but the origin of the substructures is still debated. The HL Tau system also drives a powerful bipolar wind, and we analyze its outermost component traced by CO emission, to determine the relationship of the flow with the disk and its substructures. We use ALMA observations of the ${}^{12}$CO (2-1) line at 1.3 mm, with 0.2 km/s and ~ 0.28" resolution, conducted within the ALMA-DOT project. The channel maps and position-velocity diagrams show a rich structure of concatenated bubble- and arc-shaped features, whose size and distance from the source increase with velocity. The superposition of the features generates the apparent conical shape. The tomographic reconstruction of the morphology and kinematics of the red-shifted lobe suggests the presence of distinct nested shells having higher velocity and steeper velocity gradient for shells closer to the axis, rotating in the same sense of the disk. Such configuration can be justified by different classes of models. In this paper we compare the derived wind parameters with the predictions of magnetohydrodynamic (MHD) disk winds. Under this hypothesis, the launch radii of the three outermost shells are found to be at about the position of three adjacent dust rings in the disk at 58, 72 and 86 au. The wind may be capable of removing angular momentum from the outer disk, and we derive a magnetic lever arm of $\lambda \sim 4 - 5$, higher than that commonly adopted for MHD winds from these regions. Interpretations are discussed. The arrangement of the wind in nested shells with brighter emission rooted at the location of ring substructures could support the results of non-ideal MHD simulations according to which magnetic instabilities can generate the disk ring-gap system with a connected layered wind, alternatively to the action of yet undetected protoplanets.