Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session 6HE: Shocks in Stellar Jets |
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Sponsoring Units: HEDP HEDLA Chair: Sergey Lebedev, Imperial College Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), Promenade F |
Saturday, April 12, 2008 10:50AM - 11:15AM |
6HE.00001: Observed Hydrodynamical Properties of Stellar Jets Invited Speaker: Jets from young stars are probably the best astronomical objects for studies of supersonic fluid dynamics. These remarkable collimated flows consist of multiple bow shocks which form in response to velocity perturbations. In at least two cases, jets deflect from obstacles located along the path of the flow and produce spectacular shocked wakes and shear. The hot gas behind the shocks in stellar jets radiates optically thin forbidden lines, from which one can measure turbulent line widths, densities, ionization fractions, and temperature, and even watch how the jets and shocks evolve in real time. This talk will summarize the important properties of these flows, and present new observations from the Hubble Space Telescope and from ground-based telescopes relevant to understanding fluid dynamics of stellar jets. A variety of laboratory experiments involving the dynamics of high Mach number jets and shock waves have become increasingly relevant to our understanding of this fundamental astrophysical process. [Preview Abstract] |
Saturday, April 12, 2008 11:15AM - 11:40AM |
6HE.00002: Turbulence, Outflows and Feedback Invited Speaker: In this talk I review progress on the study of protostellar outflows altering the environment into which they propagate. Using high resolution simulations we explore the ability of outflows to generate and maintain turbulence in the clouds from which they were born. In addition we explore the nature of the jets themselves in terms of new models in which the jets are, essentially, composed of a series of clumps of various densities propagating with a distribution of speeds through a background, inter-clump media. These models are supported both by observations and recent laboratory experiments. [Preview Abstract] |
Saturday, April 12, 2008 11:40AM - 12:05PM |
6HE.00003: Experiments with supersonic plasma jets at Omega Invited Speaker: Large-scale directional outflows of supersonic plasma, also known as jets, are often encountered in astrophysics. These jets propagate through the interstellar medium which is often clumpy and where inhomogeneities affect the morphology of the shocks that are generated. The hydrodynamics is difficult to model as the problem is inherently 3D, and the clumps are subject to a variety of fluid instabilities as they are accelerated and destroyed by shocks. Very large scale inhomogeneities may result in deflection of the jet itself. The traditional approach to understanding such phenomena is through theoretical analysis and numerical simulations. However, such numerical simulations have limited resolution, often assume axial symmetry, do not include all relevant physical processes, and may fail to scale correctly in Reynolds number and other key dimensionless parameters. They are frequently not tested by comparison with laboratory experiments. In recent years, we have carried out experiments at the University of Rochester's Omega laser, to investigate the physics associated with the propagation of plasma jets and shocks through both homogeneous and inhomogeneous media. These experiments have close analogues with structures observed in jets from young stars. Jets and shocks are created in experimental assemblies that are ablatively driven by a 190-eV temperature `hohlraum' (which is itself heated by the Omega laser), and subsequently propagate into a low density hydrocarbon-foam medium. The foam is either of uniformly low density, or contains localised (higher density) perturbations. Interaction of jets with this fluid results in the development of a bow shock, and, in the case of a single density perturbation, results in deflection of the jet (a laboratory analogue of the astrophysical object HH110). In the case of a shock propagating through an inhomogeneous medium (foam containing one or more sapphire spheres), the resulting complex shock interactions are analogous to the flow of clumpy interstellar matter through the working surfaces of HH objects. The hydrodynamic structures that develop in these experiments are revealed by x-ray `backlighting' radiography. These complex experimental data challenge both astrophysical and laser-plasma hydrodynamics computer codes. We discuss 2D and 3D simulations of these experiments, and their potential scaling to astrophysical systems. [Preview Abstract] |
Saturday, April 12, 2008 12:05PM - 12:30PM |
6HE.00004: Experiments with laser driven plasma jets Invited Speaker: Laboratory studies can address issues relevant to astrophysics$^{1}$ and in some cases improve our understanding of the physical processes that occur in astrophysical objects. So issues related to the jet propagation and collimation over considerable distance and their interactions with surrounding media have begun to be addressed these last years. Laboratory plasmas and astrophysical objects have different length, time and density scales. However, the typical velocities are the same, of a few hundred km/s and the similarity criteria$^{2}$ can be applied to scale the laboratory jets to astrophysical conditions. In this presentation, we use a method of jet formation$^{3}$ which allows to launch a very fast jet having a velocity around 400 km/s by using a relatively small laser energy, of the order of 100 J. The jet has a Mach number greater than 10, a length of a few mm, and a radius of a few tenths of mm. The interaction of these jets with a gas puff has been recently studied in an experiment carried out at the PALS laser facility. Varying gas pressure and composition, we show that the nature of interaction zone changes from a quasi adiabatic outflow to a strongly radiatively cooling jet. The use of various diagnostics, allows to relate the x-ray emission to the density map of the interaction zone. Already observed in astrophysical objets for strongly different time and space scales, these structures are interpreted in our laboratory experiment by using a semi-analytical model and 2D radiation hydrodynamic simulations. [1] B. Remington et al, Rev. Mod. Phys. 78, 755 (2007) [2] D. Ryutov et al, Phys~. Plasmas 8, 1804 (2001) [3] Ph. Nicolai et al, Phys. Plasmas 13, 062701 (2007) [Preview Abstract] |
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