Machine-gun jets from time-dependent sources

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Abstract
The sources of astrophysical jets (e.g. active galactic nuclei and young stellar objects) often appear to have complex, time-dependent properties. This leads us to expect that the jet flows are likely to be ejected with a time-dependent velocity and/or direction of ejection. In this paper, we study the coupled effect of velocity and direction time variabilities of the jet source. We show that such time variabilities have the effect of breaking up the jet into high-density clumps (or ‘bullets’), which travel away from the source in different directions. This model appears to be attractive for the explanation of astrophysical outflows that have a number of discrete, disconnected clumps rather than a continuous, jet-like structure. An application of these models to the case of Herbig–Haro objects is discussed. The models presented in this paper apply to ‘heavy’ jets (i.e. denser than the surrounding environment). This of course limits the application of these models to extragalactic jets, which are most likely to be ‘light’ jets (i.e. with densities lower than the surrounding environment). These models can, however, be applied to describe jet beams surrounded by hot, low-density ‘cocoons’, a regime which is probably relevant for many extragalactic jets.

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