ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

Abstract

Context . The ALMA image of the ringed disk around HL Tau stands out as the iconic signature of planet formation, but the origin of the observed substructures is still debated. The HL Tau system also drives a powerful bipolar wind, detected in atomic and molecular lines, that may have important feedback on the process. Aims . The outermost component of the wind traced by CO emission was analyzed in detail to determine its relationship with the disk and its substructures. Methods . A spectro-imaging investigation was conducted using ALMA observations of the 12 CO (2-1) line at 1.3 mm, with 0.2 km s −1 and ~0 ⋅ ′′28 spectral and angular resolution, in the framework of the ALMA-DOT project. The relevant wind parameters were derived, allowing a tomographic reconstruction of the morphology and kinematics of the redshifted lobe of the outflow to be compared with theoretical models. Results . The data channel maps and position-velocity diagrams show a rich substructure of concatenated bubble- and arc-shaped features, whose size and distance from the source continuously increase with velocity. The superposition of such features generates the apparent conical shape. The spatial-kinematic properties suggest that the flow presents distinct nested shells with higher propagation speed and steeper speed gradient with distance from the source for shells progressively closer to the axis, and rotating in the same direction as the disk. The wind parameters were compared with the predictions of magnetohydrodynamic (MHD) disk winds. Under this hypothesis, the launch radii of the three outermost shells are found to correspond to the positions of three rings in the dust emission distribution of the disk located at 58, 72, and 86 astronomical units from the star. We derive a magnetic lever arm λ ~ 4–5, higher than that commonly adopted in models of MHD winds from the outer disk. Interpretations are discussed. Conclusions . The properties of the CO outflow from HL Tau appear to be compatible with magnetized disk winds with launch radii in the region at 50–90 au from the source. As such, the wind may be capable of removing angular momentum also from the outer disk. The arrangement of the wind in nested shells with brighter emission rooted at the location of ring substructures could support the results of recent non-ideal MHD simulations according to which magnetic instabilities can spontaneously generate both the ring–gap system and a connected inhomogeneous layered wind, alternatively to the action of protoplanets. Further observational analyses and comparisons with other classes of models will help establish the role of magnetic effects in the process of planet formation.

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