HEAVY, OVERPRESSURED JETS AND THE RADIO-SOURCE HERCULES-A

We investigate the flow of jets with pressure and density initially much greater than the medium into which they are injected. While having many features in common with the classical light, pressure-balanced jets studied in recent investigations, heavy, overpressured jets differ in that they expand at the Mach angle and have a conical expansion shock and large convex Mach disk. The ''ice cream cone'' shape of these shocks should be visible in high-dynamic-range radio maps. Some numerical models are observed to recollimate, creating broader jets at lower density. We develop analytic expressions which describe the cocoon formation and recollimation processes. The concept of a jet with a large overpressure should be applicable to many situations where the observed jet has a conical appearance and, perhaps, recollimates. Under the assumption that the rings seen in high-resolution images of Hercules A are shocks, we model the western lobe of this source as a heavy overpressured flow where, about 20'' from the nucleus, the jet suddenly becomes much greater in pressure than the surrounding medium and freely expands. We argue that jet models which are supersonic with respect to both the internal and external speeds are applicable and find that, in spite of the prominent ring structure in Her A, the flow must be dominated by a smooth conical jet. This result is consistent with the kinematic model of Morrison, & Sadun. Although we do not numerically simulate the rings in the western lobe, our models are also consistent with MMS suggestion that the rings are weak spherical shocks in that flow: their size is small compared to the scale length for the sound speed to change. Depending on the exact flow velocity and Mach number, the jet may be initially heavy (eta-o > 1) compared to the ambient medium at the point where expansion begins. The flow becomes light (eta > 6 x 10(-3)) much further out due to the expansion and is able to produce the cocoon seen in low-resolution radio maps. We have tentatively identified the Mach disk, bow shock, and the plug in the radio map of the western lobe as the two outer arcs of emission and the space between them, respectively. The identification of these arcs as strong shocks is strengthened by their observed magnetic field structure. Additional studies are needed to understand ring formation and propagation in such flows and to understand why the sudden expansion in Her A takes place. The former problem can be addressed with detailed, high-resolution numerical studies of perturbed heavy, overpressured jets. The latter should benefit from high-resolution X-ray imaging of the inner arcminute of the Her A region to understand better the structure of the ambient medium and to determine what influence, if any, it may have on the jet flow.