Outflows from young stellar objects: the case of DG Tauri

Abstract

Protostellar jets and winds play a crucial role in the dynamics and evolution of the starformation process. They may effectively regulate mass accretion by removing angular momentum from the circumstellar disc. Despite their importance, the physical processes driving the outflow phenomena remain poorly understood. This thesis presents a consistent model for the outflow structure and dynamics of the young stellar object DG Tauri, using data of unprecedented spatial and spectral resolution from the Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The approaching outflow shows two components in [Fe II] 1.644 m emission. A stationary recollimation shock is observed in the high-velocity jet, in agreement with previous Xray and FUV observations. The pre-shock jet velocity, and inferred jet launch point (400-700 km s-1 and 0.02-0.07 AU, respectively), are signifcantly different from previous estimates. Jet `acceleration' beyond the shock is interpreted as intrinsic velocity variability. Careful analysis reveals no evidence of jet rotation, contrary to previous work. A wide-angle, low-velocity blueshifted molecular out ow is observed in H2 1-0 S(1) 2.1218 m emission. Both outflows are consistent with a magnetocentrifugal disc wind origin, although an X-wind origin for the jet cannot be excluded. The lower-velocity [Fe II] component surrounds the jet, and is interpreted as a turbulent mixing layer generated by lateral jet entrainment of molecular wind material. An analytical model of an entrainment layer is constructed, based on Riemann decomposition of directly observable outflow parameters. The model reproduces the velocity field of the entrained material without invoking an arbitrary `entrainment efficiency' parameter. The luminosity and mass entrainment rate estimated using the model are in agreement with observations. Such lateral entrainment requires a magnetic field strength of order a few mG at hundreds of AU above the disc surface; independent arguments are advanced to support this conclusion. The receding outflow of DG Tau takes on a bubble-shaped morphology. Kinetic models indicate this structure is a quasi-static bubble with an internal velocity field describing expansion. It is proposed that this bubble forms because the receding counterjet from DG Tau is obstructed by a clumpy ambient medium. There is evidence of interaction between the counterjet and ambient material, which is attributed to the large molecular envelope around the DG Tau system. An analytical model of a momentum-driven bubble is shown to be consistent with observations. It is concluded that the bipolar outflow from DG Tau is intrinsically symmetric; the observed asymmetries are due to environmental effects. The observational interpretations and comprehensive modelling of the DG Tau outflows presented in this thesis constitute a significant step forward in gaining a full physical understanding of how stars accrete their mass. The complex nature of the approaching jet provides the first clear indications of the diverse phenomena associated with protostellar mass loss. The different morphology of the receding out ow has highlighted the role of environmental factors in defining outflow characteristics. Together this work presents a new and more detailed view of the complex mechanisms associated with the formation of a low-mass star.