3-D Simulations of Protostellar Jets in Stratified Ambient Media
  We present fully three-dimensional hydrodynamical simulations of radiative cooling jets propagating into stratified isothermal ambient media with power-law density and pressure distributions. The parameters used are mainly suitable for protostellar jets but results applicable to extragalactic jets are also presented. Comparisons are made with previous simulations of jets through homogeneous media. We find that for radiative cooling jets propagating into regions where the ambient medium has an increasing density (and pressure) gradient, the ambient gas tends to compress the cold, low-pressure cocoon of shocked material that surrounds the beam and destroy the bow shock-like structure at the head. The compressing medium collimates the jet and promotes the development of Kelvin-Helmholtz instabilities which cause beam focusing, wiggling and the formation of internal traveling shocks, $close$ $to$ $the$ $head$, via pinching along the beam. This remarkably resembles the structure of some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects are larger for jets with smaller density ratio between jet and environment $\eta $ (tested for $\eta $=1, 3, and 10) and larger Mach number $M_a=v_j/c_a$ (tested for $M_a=$12 and 24, where $v_j$ is the jet velocity and $c_a$ the ambient sound speed). In an ambient medium of decreasing density (and pressure), the beam is poorly collimated and relaxes, becoming faint. This could explain ''invisible'' jet sections, like the gap between the parent source and collimated beam (e.g., in HH30 jet). Although, on average, jets propagating into an increasing (decreasing) density environment are decelerated (accelerated) by the increasing (decreasing) ram pressure of the ambient medium, we find that their propagation velocities have an oscillating pattern. 
Elisabete M. de Gouveia Dal Pino and Mark Birkinshaw