Hypersonic Buckshot: Astrophysical Jets as Heterogeneous Collimated Plasmoids

Abstract

Herbig-Haro jets are commonly thought of as homogeneous beams of plasma traveling at hypersonic velocities. Structure within jet beams is often attributed to periodic or "pulsed" variations of conditions at the jet source. Simulations based on this scenario result in knots extending across the jet diameter. Observations and recent high energy density laboratory experiments shed new light on structures below this scale and indicate they may be important for understanding the fundamentals of jet dynamics. In this paper, we offer an alternative to "pulsed" models of protostellar jets. Using direct numerical simulations we explore the possibility that jets are chains of subradial clumps propagating through a moving interclump medium. Our models explore an idealization of this scenario by injecting small (r < r jet ), dense (ρ > ρ jet ) spheres embedded in an otherwise smooth interclump jet flow. The spheres are initialized with velocities differing from the jet velocity by ∼15%. We find that the consequences of shifting from homogeneous to heterogeneous flows are significant as clumps interact with each other and with the interclump medium in a variety of ways. Structures which mimic what is expected from pulsed-jet models can form, as can be previously unseen, "subradial" behaviors including backward facing bow shocks and off-axis working surfaces. While these small-scale structures have not been seen before in simulation studies, they are found in high-resolution jet observations. We discuss implications of our simulations for the interpretation of protostellar jets with regard to characterization of knots by a "lifetime" or "velocity history" approach as well as linking observed structures with central engines which produce the jets.

A variable jet model for the Hαemission of HH 444

“…These knots can in principle be modeled through the introduction of an extra ejection variability mode (a similar morphology in the HH 34 jet was modeled in this way by Raga & Noriega-Crespo 1998). Instead of a multi-mode variability, a non-periodic variability (see Yirak et al 2009;Bonito et al 2010) might be present, but the richness of inter-knot spatial scales that is to be expected from a well-sampled random variability (Raga 1992) does not seem to be present in the HH 444 jet.…”

“…In order to obtain the correct knot separations it will be necessary to consider at least a two-mode ejection variability model (like the one explored by Raga & Noriega-Crespo 1998), or possibly a non-periodic ejection variability (like the one recently explored by Yirak et al 2009). A two-mode ejection variability model is presented in Sect.…”

A study of the wiggle morphology of HH 211 through numerical simulations

“…The sub-knots may eventually converge into the larger knots seen downstream (e.g. Yirak et al 2009;Raga et al 2012). This was postulated by Lee et al (2010) to account for wide velocity range detected in Figure 3.…”

STIS optical spectroscopy of the lobes of CRL 618

“…It is not clear, however, how the "clumps model" can reproduce the observed excitation conditions. Recently, Yirak et al (2009Yirak et al ( , 2011 proposed the "clumped" jet model to reproduce the overall morphology of the HH jets. They assume the presence of clumps within the jet beam with random locations, sizes and velocities.…”