Three-dimensional Simulations of Jet/Cloud Interactions: Structure and Kinematics of the Deflected Jets

We report the results of three-dimensional smoothed particle hydrodynamics simulations of interactions of overdense, radiatively cooling and adiabatic jets with dense, compact clouds in frontal and off-axis collisions. Calculated for a set of parameters that are particularly appropriate to protostellar jets, our results indicate that the interaction produces important transient and permanent effects in the jet morphology. In off-axis interactions, the deflected beam initially describes a C-shaped trajectory around the curved jet/cloud contact discontinuity, but the deflection angle tends to decrease with time as the beam slowly penetrates the cloud. Later, when the jet has penetrated most of the cloud extension, the deflected beam fades and the jet resumes its original direction of propagation. During the interaction, a weak chain of internal knots develops along the deflected beam and the velocity field initially has a complex structure that later evolves to a more uniform distribution. The average velocity of the deflected beam is consistent with the predicted value given by v ≃ vj cos θ (where θ is the deflection angle and vj is the velocity of the incident beam). The impact also decreases the beam collimation. Applied to the context of the protostellar jets, this morphology and kinematics found for the deflected beam is very similar to that observed in some candidate systems like the HH 110 jet, which has been previously proposed to be the deflected part of the HH 270 jet. Our simulations also reveal the formation of a head-neck bright structure at the region of impact that resembles the morphology of the HH 110 knot A located in the apex of the HH 110 jet, where the deflection is believed to occur. All these similarities strongly support the proposed jet/cloud interaction interpretation for this system. The fact that the deflection angles derived from the simulations are smaller than that observed and the fact that the jet/cloud interaction is still taking place indicate that the interacting cloud in that system must have a radius Rc ≫ Rj, where Rj is the jet radius, as previously suggested, and a density ratio between the jet and the cloud β2 = nj/nc ≲ 10-2. Because of the small size of the clouds [with radius Rc ≃ (1-2)Rj], the interactions examined here are very transient (with lifetimes of few ~10 to ~100 yr that are much less than the typical dynamical lifetimes of the protostellar outflows, τ ≳ 104 yr). Nonetheless, they leave important signatures in the surviving outflow. The leftovers of the cloud and the knots that are produced in the deflected beam are deposited into the working surface and contribute to enrich the knotty pattern commonly observed in Herbig-Haro objects behind the bow shocks of protostellar jets. Also, the collision may partially destroy the shell at the head, producing remarkable asymmetries in the head region. A jet undergoing many transient interactions with compact clumps along its propagation and lifetime may inject a considerable amount of shocked jet material sideways into the surrounding ambient medium, and this process may provide a powerful tool for momentum transfer and turbulent mixing with the ambient medium.