HOLLOW CONICAL JET MODELS FOR SS 433 - A PARADIGM LOST

A precessing jet can be approximated as an axisymmetric flow if the precession time is short compared to the propagation time over scales of interest. An example is the jet in SS 433, where the precession time is ≃0.5 yr and the propagation time from SS 433 to the W50 remnant is ≳1000 yr. We perform a series of sunulations of precessing jets using a R - Z (axisymmetric) finite difference hydrodynamics code. We reproduce first the results for the filled jets studied previously by several groups. Next, we examine hollow cylindrical jets which allow us to examine the effects of a hollow jet without the complications of a growing interior volume. This case may serve as a "postfocusing" model of a precessing jet. Finally, we examine hollow conical jets, which model the behavior of a precessing jet propagating on the surface of its precession cone. The conical jets "stall" at the point where the momentum flux density in the jet becomes too low to push the ambient gas from its path. If the area of the working surface over which the jet transfers momentum to the ambient medium is Aw, the area of the jet orifice is Aj, and the initial internal Mach number of the jet is Mj, the jet stalls when Aw = M2jAj. The jet propagates in an extremely unsteady fashion, with large radial oscillations in the position of the jet's channel. The oscillations cause Aw to grow proportional to R2, where R is the outer radius of the jet, so that the jet stalls at R ≃ MjR0, where R0 is the initial outer radius of the jet. The oscillations of the jet are caused by the competition between the supersonic vortices, which make up the cocoon about the exterior of the jet, and the shocked ambient gas on the interior of the jet. While the cocoon can induce a temporary focusing of the jet, the pressure in the interior is able to prevent any permanent focusing. The bow shocks of the hollow jets are much flatter than those of the filled jets, and no feature which resembles the "ears" of the W50 remnant develops. It seems unlikely that the "ears" of W50 can be formed by a hydrodynamic jet ejected with a precession cone as large as that currently observed in the SS 433 system.