Abstract
We present a thin shell, analytic model for an outflow ejected with an opening angle that increases as a function of time. This model predicts a cavity shape that broadens out with time, and pinches towards the outflow axis at a point that travels away from the outflow source. The motivation for this model is the set of observations of a bipolar cavity centered on Barnard 5 IRS 1 by Velusamy & Langer (1998), who suggest an interpretation for the observed structure in terms of a model such as the one that we have studied. We find that this observed structure could indeed be produced by an outflow of decreasing collimation as a function of time, provided that the mass loss rate has a value dot M0 ≈ 3×10-6 M⊙ yr-1.
