8 research outputs found
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- 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 SmallâScale 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 (),
dense () spheres embedded in an otherwise smooth inter-clump
jet flow. The spheres are initialized with velocities differing from the jet
velocity by %. 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 scalesâfrom 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 simulationsâemploying adaptive mesh
refinement (AMR) with the AstroBEAR codeâseeking 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 jetsâ 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 âclumpiness,â agreement with observations is
found, primarily in morphological and kinematic signatures. Finally, the âclumpy jetâ
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