Outflow-driven Cavities: Numerical Simulations of Intermediaries of Protostellar Turbulence

We investigate the evolution of fossil cavities produced by extinct young stellar object (YSO) jets and wide-angle outflows. Fossil cavities are ellipsoidal or cylindrical shells of swept-up ambient (molecular cloud) material moving at low velocities. The cavities form when the momentum in a YSO jet or wide-angle outflow decays in time, allowing the bow shock or swept-up shell to decelerate to velocities near the turbulent speed in the cloud. It has been suggested in previous studies that cavities provide efficient coupling between the jets/outflows and the cloud and, as such, are the agents by which cloud turbulence can be re-energized. In this paper, we carry forward a series of numerical simulations of jets and outflows whose momentum flux decreases in time. We compare simulations with decaying momentum fluxes to those with constant flux. We show that decaying flux models exhibit deceleration of the outflow head and back-filling via expansion off of the cavity walls. They also have lower density contrast and are longer lived and wider than their continuously driven counterparts. The simulations recover the basic properties of observed fossil cavities. In addition, we provide synthetic observations in terms of position-velocity (PV) diagrams, which demonstrate that fossil cavities form both jets and wide-angle outflows and are characterized by linear "Hubble law" expansion patterns superimposed on "spur" patterns, indicative of the head of a bow shock.