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

Abstract

Context. 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. Aims. 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. Methods. Our analysis is based on Atacama Large Millimeter Array (ALMA) 12CO(2–1) observations of DG Tau B at 0.15″ (20 au) angular resolution. We developed a tomographic method to recover 2D (R,Z) maps of vertical velocity VZ and specific angular momentum j = R × Vϕ. We created synthetic data cubes for parametric models of wind-driven shells and disk winds, which we fit to the observed channel maps. Results. Tomographic analysis of the bright inner conical outflow shows that both VZ and j remain roughly constant along conical surfaces, defining a shear-like structure. 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 (λ ≤ 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. Conclusions. 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.