PROPERTIES OF SWEPT-UP MOLECULAR OUTFLOWS

We analyze models of momentum-conserving, molecular outflows swept up by radially directed stellar winds from young stellar objects, and we introduce the new constraint that the models should reproduce the observed spectral line shapes of outflows, which trace the amount of mass as a function of velocity. The models can easily reproduce the spatial shape of a typical outflow and the appearance of the highest velocity at the edge of the flow, but they have difficulty in reproducing the observed spectral fine shape. To produce the elongated spatial shapes of outflows, a very large pole/equator contrast is required in either or both of the density of the ambient medium and the pressure of the driving wind. However, the line shapes, which show very little material traveling at extreme velocities, require a nearly evacuated channel to guide the flow in the ambient medium. Such structures are not observed in molecular clouds, and we conclude that a radially driven outflow model cannot reproduce the properties of observed molecular outflows. A more promising mechanism for such outflows is one in which the initial wind from the star is a collimated jet, which sweeps aside the material in its path, entraining ambient gas in a turbulent fashion, rather than simply pushing it forward like a snowplow.