The Evolution of Hetergeneous "Clumpy Jets": A Parameter Study

We investigate the role discrete clumps embedded in an astrophysical jet play on the jet's morphology and line emission characteristics. By varying clumps' size, density, position, and velocity, we cover a range of parameter space motivated by observations of objects such as the Herbig Haro object HH~34. We here extend the results presented in Yirak et al. 2009, including how analysis of individual observations may lead to spurious sinusoidal variation whose parameters vary widely over time, owing chiefly to interacts between clumps. The goodness of the fits, while poor in all simulations, are best when clump-clump collisions are minimal. Our results indicate that a large velocity dispersion leads to a clump-clump collision-dominated flow which disrupts the jet beam. Finally, we present synthetic emission images of H-$\alpha$ and [SII] and note an excess of [SII] emission along the jet length as compared to observations. This suggests that observed beams undergo earlier processing, if they are present at all.Comment: 18 pages, 7 figures. Submitted to the Astrophysical Journa

The Interaction between a Pulsed Astrophysical Jet and Small‐Scale Heterogeneous Media

We have performed two-dimensional hydrodynamic simulations of a pulsed astrophysical jet propagating through a medium that is populated with circular inhomogeneities, or "clumps," which are smaller than the jet width. The clumps are seen to affect the jet in several ways, such as impeding jet propagation and deflecting the jet off-axis. While there has been some debate as to the prevalence of these types of condensations in the ISM or in molecular clouds, the exploration of this region of parameter space nonetheless both shows the potential for these clumps to disrupt astrophysical jets and yields results which recover aspects of recent observations of Herbig-Haro objects. We find that the propagation of the jet and the vorticity induced in the clump/ambient medium correlate well with a "dynamic filling function" fd across all the simulations

Hypersonic Buckshot: Astrophysical Jets as Heterogeneous Collimated Plasmoids

Herbig-Haro (HH) 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 sub-radial clumps propagating through a moving inter-clump medium. Our models explore an idealization of this scenario by injecting small ($r<r_{jet}$), dense ($\rho>\rho_{jet}$) spheres embedded in an otherwise smooth inter-clump jet flow. The spheres are initialized with velocities differing from the jet velocity by $\sim15$%. We find the consequences of shifting from homogeneous to heterogeneous flows are significant as clumps interact with each other and with the inter-clump medium in a variety of ways. Structures which mimic what is expected from pulsed-jet models can form, as can previously unseen ``sub-radial'' 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.Comment: 15 pages, 3 figures (1 color), submitted to Ap

Heterogeneous interactions in the interstellar medium

Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2010.Heterogeneous processes exist on a variety of astrophysical scales—from galaxies, to star-forming regions, to stars themselves. Such heterogeneous (clumpy) processes are a rich area of investigation. The Hubble Space Telescope (HST) has opened up a window to, for example, the small-scale heterogeneity of jet-like Herbig-Haro (HH) objects. Complementing this, the vast improvements of computing capability over the past several decades have allowed theory-driven direct numerical simulation to thrive. The present work is the result of several sets of simulations—employing adaptive mesh refinement (AMR) with the AstroBEAR code—seeking to address questions related to clumpy astrophysical jets. While their morphology typically is ascribed to a periodic or otherwise smoothly-time-varying launching engine, two alternatives are proposed. The first examines the role of heterogeneity in the jets’ environment. Several important correspondences between the simulations and observations are found. Conversely, a model is proposed in which the jets themselves are heterogeneous. Via a study of parameter space based on the degree of “clumpiness,” agreement with observations is found, primarily in morphological and kinematic signatures. Finally, the “clumpy jet” model brings to light questions concerning the clumps themselves. Specifically, how the concept of sufficient resolution needs to be modified when the additional physical process of radiative cooling is included. Radiative cooling removes energy from these systems primarily from shock-heated gas. Since many observations derive from the same shock-heating mechanisms, correct modelling when radiative cooling is included is very important. This question is addressed with a suite of simulations which cover several decades in resolution. Finally, a new criterion is proposed to take into account the role of radiative cooling when AMR is employed