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. Calcul ated for a set of parameters that are particularly appropriate to protostel lar jets, our results indicate that the interaction produces important tran sient and permanent effects in the jet morphology. In off-axis interactions , the deflected beam initially describes a C-shaped trajectory around the c urved jet/cloud contact discontinuity, but the deflection angle tends to de crease with time as the beam slowly penetrates the cloud. Later, when the j et has penetrated most of the cloud extension, the deflected beam fades and the jet resumes its original direction of propagation. During the interact ion, a weak chain of internal knots develops along the deflected beam and t he velocity field initially has a complex structure that later evolves to a more uniform distribution. The average velocity of the deflected beam is c onsistent with the predicted value given by upsilon(j)' similar or equal to upsilon(j) cos theta (where theta is the deflection angle and upsilon(j) i s the velocity of the incident beam). The impact als:, decreases the beam c ollimation. Applied to the context of the protostellar jets, this morpholog y and kinematics found for the deflected beam is very similar to that obser ved in some candidate systems like the HH 110 jet, which has been previousl y 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 impa ct that resembles the morphology of the HH 110 knot A located in the apex o f the HH 110 jet, where the deflection is believed to occur. All these simi larities strongly support the proposed jet/cloud interaction interpretation for this system. The fact that the deflection angles derived from the simu lations are smaller than that observed and the fact that the jet/cloud inte raction is still taking place indicate that the interacting cloud in that s ystem must have a radius R-c much greater than R-j, where R-j is the jet ra dius, as previously suggested, and a density ratio between the jet and the cloud beta(2) = n(j)/n(c) less than or similar to 10(-2). Because of the sm all size of the clouds [:with radius R-c similar or equal to (1-2)R-j], the interactions examined here are very transient (with lifetimes of few simil ar to 10 to similar to 100 yr that are much less than the typical dynamical lifetimes of the protostellar outflows, tau greater than or similar to 10( 4) yr). Nonetheless, they leave important signatures in the surviving outfl ow. The leftovers of the cloud and the knots that are produced in the defle cted beam are deposited into the working surface and contribute to enrich t he knotty pattern commonly observed in Herbig-Haro objects behind the bow s hocks of protostellar jets. Also, the collision may partially destroy the s hell at the head, producing remarkable asymmetries in the head region. A je t undergoing many transient interactions with compact clumps along its prop agation and lifetime may inject a considerable amount of shocked jet materi al sideways into the surrounding ambient medium, and this process may provi de a powerful tool for momentum transfer and turbulent mixing with the ambi ent medium.