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The origin of bipolar outflow asymmetry in young stellar objects (YSOs)remains poorly understood. It may be due to an intrinsically asymmetric outflowlaunch mechanism, or it may be caused by the effects of the ambient mediumsurrounding the YSO. Answering this question is an important step inunderstanding outflow launching. We have investigated the bipolar outflowsdriven by the T Tauri star DG Tauri on scales of hundreds of AU, using theNear-infrared Integral Field Spectrograph (NIFS) on Gemini North. Theapproaching outflow consists of a well-collimated jet, nested within alower-velocity disc wind. The receding outflow is composed of asingle-component bubble-like structure. We analyse the kinemat- ics of thereceding outflow using kinetic models, and determine that it is aquasi-stationary bubble with an expanding internal velocity field. We proposethat this bubble forms because the receding counterjet from DG Tau isobstructed by a clumpy ambient medium above the circumstellar disc surface,based on similarities between this structure and those found in the modeling ofactive galactic nuclei outflows. We find evidence of interaction between theobscured counterjet and clumpy ambient material, which we attribute to thelarge molecular envelope around the DG Tau system. An analytical model of amomentum-driven bubble is shown to be consistent with our interpretation. Weconclude that the bipolar outflow from DG Tau is intrinsically symmetric, andthe observed asymmetries are due to environmental effects. This mechanism canpotentially be used to explain the observed bipolar asymmetries in other YSOoutflows.

Multi-epoch Sub-arcsecond [Fe II] Spectroimaging of the DG Tau Outflows with NIFS. II. On the Nature of the Bipolar Outflow Asymmetry