THE STRUCTURE AND KINEMATICS OF BIPOLAR OUTFLOWS - OBSERVATIONS AND MODELS OF THE MONOCEROS R2 OUTFLOW

We present new detailed (CO)-C-12 observations of the giant bipolar molecular outflow associated with the Mon R2 infrared cluster. These observations suggest that the outflow consists of two distinct pairs of bipolar lobes, which partially overlap. The observed velocity field in two of the four lobes is well characterized by a linear increase of projected flow velocity with projected distance from the flow center. We find strong evidence for shell structure in the lobes, and also that the collimation of the outflow appears to increase both with increasing distance from the flow center and with increasing velocity of the outflowing gas. To try to understand the spatial and velocity structure of this outflow we have developed new models of the structure and kinematics of highly collimated outflow sources. Our models indicate that the extent to which highly collimated flows are able to appear bipolar on the sky depends predominantly on the structure of the velocity field of the outflowing gas. The high frequency of bipolarity among known outflow sources is a strong constraint which requires the velocity of outflowing gas to be directed predominantly along the axis of the outflow. We find two of the outflow lobes in Mon R2 are very well fitted by a paraboloidal shell model. In particular, the model predicts line splitting in the outflow lobes which has the same extent and symmetry about the origin of the flow as is observed. The model which best fits the velocity structure of the Mon R2 outflow is one whose velocity field is characterized by a "Hubble"-like or upsilon-proportional R velocity power law. It is physically implausible to produce such a power law solely by in situ acceleration of material away from the origin of the flow. Instead, velocity-sorting of initially clumpy flow gas, resulting from entrainment by a wind, or possibly by an explosive event, probably gives rise to the observed linear velocity power law. Finally, the inability of the simple flow models to account for the dual spatial structure observed in each of the Mon R2 outflow lobes suggest that the Mon R2 flow consists of two physically distinct bipolar outflows whose major axes are centered on the infrared cluster but are roughly orthogonal.