  The origin of bipolar outflow asymmetry in young stellar objects (YSOs)
remains poorly understood. It may be due to an intrinsically asymmetric outflow
launch mechanism, or it may be caused by the effects of the ambient medium
surrounding the YSO. Answering this question is an important step in
understanding outflow launching. We have investigated the bipolar outflows
driven by the T Tauri star DG Tauri on scales of hundreds of AU, using the
Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The
approaching outflow consists of a well-collimated jet, nested within a
lower-velocity disc wind. The receding outflow is composed of a
single-component bubble-like structure. We analyse the kinemat- ics of the
receding outflow using kinetic models, and determine that it is a
quasi-stationary bubble with an expanding internal velocity field. We propose
that this bubble forms because the receding counterjet from DG Tau is
obstructed by a clumpy ambient medium above the circumstellar disc surface,
based on similarities between this structure and those found in the modeling of
active galactic nuclei outflows. We find evidence of interaction between the
obscured counterjet and clumpy ambient material, which we attribute to the
large molecular envelope around the DG Tau system. An analytical model of a
momentum-driven bubble is shown to be consistent with our interpretation. We
conclude that the bipolar outflow from DG Tau is intrinsically symmetric, and
the observed asymmetries are due to environmental effects. This mechanism can
potentially be used to explain the observed bipolar asymmetries in other YSO
outflows.
