Modeling the CO outflow in DG Tau B: Swept-up shells versus perturbed MHD disk wind

TL;DR

This study uses high-resolution ALMA observations to analyze the morphology and kinematics of the DG Tau B outflow, providing evidence for a massive MHD disk wind influencing disk evolution and early planet formation.

Contribution

It demonstrates that a steady conical MHD disk wind explains the observed outflow structures better than wind-driven shell models, highlighting the role of magnetic winds in early star and planet formation.

Findings

Conical MHD wind models fit the observed outflow structures.

The wind mass flux exceeds the accretion rate, indicating significant angular momentum removal.

Evidence suggests dynamic environment influences early planet formation.

Abstract

The origin of outflows and their exact impact on disk evolution and planet formation remain crucial open questions. DG Tau B is a Class I protostar associated with a rotating conical CO outflow and a structured disk. Hence it is an ideal target to study these questions. We aim to characterize the morphology and kinematics of the DG Tau B outflow in order to elucidate its origin and potential impact on the disk. Our analysis is based on Atacama Large Millimeter Array (ALMA) 12CO(2-1) observations of DG Tau B at 20 au angular resolution. We characterize three different types of substructures in this outflow (arches, fingers, and cusps) with apparent acceleration. Wind-driven shell models with a Hubble law fail to explain these substructures. In contrast, both the morphology and kinematics of the conical flow can be explained by a steady conical magnetohydrodynamic (MHD) disk wind with foot-point radii r0= 0.7-3.4 au, a small magnetic level arm parameter lambda < 1.6), and quasi periodic brightness enhancements. These might be caused by the impact of jet bow shocks, source orbital motion caused by a 25 MJ companion at 50 au, or disk density perturbations accreting through the wind launching region. The large CO wind mass flux (four times the accretion rate onto the central star) can also be explained if the MHD disk wind removes most of the angular momentum required for steady disk accretion. Our results provide the strongest evidence so far for the presence of massive MHD disk winds in Class I sources with residual infall, and they suggest that the initial stages of planet formation take place in a highly dynamic environment.