Three‐Dimensional Simulations of Protostellar Jets in Stratified Ambient Media

Abstract

We present fully three-dimensional hydrodynamic 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., the Haro 6-5B northern and HH 24G jets). These effects are larger for jets with a smaller density ratio η between jet and environment (tested for η = 1, 3, and 10) and larger Mach numbers Ma = vj/ca (tested for Ma = 12 and 24, where vj is the jet velocity and ca is 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 the HH 30 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 oscillatory pattern. The internal traveling shocks that develop in jets propagating into positive density gradient environments display a similar velocity variation, in qualitative agreement with recent measurements of fluctuations in the tangential velocity of the knots of the Haro 6-5B jet. Finally, runs of adiabatic jets into similar stratified environments indicate that they are less affected by the effects of stratification than the cooling jets because their higher pressure cocoons are better able to preserve the beam structure.