Magnetocentrifugally-Driven Winds from Rapidly Rotating Protostars

We examine the hypothesis that bipolar outflows associated with deeply embedded young stellar objects are driven by winds from rapidly rotating magnetized protostars. We begin by summarizing the observed properties of outflows and related phenomena and the constraints that they place on the possible physical mechanisms responsible for these phenomena. The various physical mechanisms that have been proposed to explain these phenomena are then discussed and compared in the context of these observational constraints. We pay particular attention to the X-celerator mechanism in which mass loss along open magnetic field lines emerging from the equatorial regions of a critically rotating star is centrifugally enhanced by the combination of rapid rotation and strong magnetic fields. In order to investigate the X-celerator mechanism in greater detail and to enable quantitative observational predictions, we construct detailed 2D steady state axisymmetric models of the sub-Alfvenic regions of winds driven by the X-celerator mechanism. Explicit consideration of the 2D nature of the flow enables us to study the ability of the X-celerator mechanism to collimate the flow. We find that smooth acceleration to speeds of the order of the escape speed accompanied by substantial collimation of the flow can be achieved within a few stellar radii. Applying these calculations to the protostellar case, we show that at mass loss rates of 10^{-6} M_odot y^{ -1}, the X-celerator mechanism can accelerate protostellar winds to velocities of 100s km s^ {-1} within a few stellar radii given kilogauss strength stellar magnetic fields. We discuss the less restrictive conditions under which the X-celerator mechanism may operate and compare our results to the predictions of other magneto-centrifugally driven stellar wind models. We conclude with a discussion of further possible applications of the results presented in this study. In particular, we discuss (i) observational diagnostics of the protostellar wind acceleration region and the possibility of observing kinematic signatures of the X-celerator mechanism, (ii) the implications of these results for thermal and chemical models of protostellar winds, and (iii) the possible application of the X-celerator mechanism to winds from evolved stars.