Three‐Dimensional Simulations of Protostellar Jets in Stratified Ambient Media

We present fully three-dimensional hydrodynamical simulations of radiativecooling jets propagating into stratified isothermal ambient media withpower-law density and pressure distributions. The parameters used are mainlysuitable for protostellar jets but results applicable to extragalactic jets arealso presented. Comparisons are made with previous simulations of jets throughhomogeneous media. We find that for radiative cooling jets propagating intoregions where the ambient medium has an increasing density (and pressure)gradient, the ambient gas tends to compress the cold, low-pressure cocoon ofshocked material that surrounds the beam and destroy the bow shock-likestructure at the head. The compressing medium collimates the jet and promotesthe development of Kelvin-Helmholtz instabilities which cause beam focusing,wiggling and the formation of internal traveling shocks, $close$ $to$ $the$$head$, via pinching along the beam. This remarkably resembles the structure ofsome observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effectsare larger for jets with smaller density ratio between jet and environment$\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 $c_a$ theambient sound speed). In an ambient medium of decreasing density (andpressure), the beam is poorly collimated and relaxes, becoming faint. Thiscould explain ''invisible'' jet sections, like the gap between the parentsource and collimated beam (e.g., in HH30 jet). Although, on average, jetspropagating into an increasing (decreasing) density environment are decelerated(accelerated) by the increasing (decreasing) ram pressure of the ambientmedium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the Astrophys. J., vol 471, nov. 10t