Browsing by Author "Frank, A."

Now showing 1 - 3 of 3
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    Cooling and instabilities in colliding flows
    (Oxford University Press, 2021) Markwick, R.N.; Frank, A.; Carroll-Nellenback, J.; Liu, B.; Blackman, E.G.; Lebedev, S.V.; Hartigan, P.M.
    Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power-law cooling function. The characteristic length and time-scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently and rapidly, the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one- and three-dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period.
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    Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet
    (Springer Nature, 2016) Li, C.K.; Tzeferacos, P.; Lamb, D.; Gregori, G.; Norreys, P.A.; Rosenberg, M.J.; Follett, R.K.; Froula, D.H.; Koenig, M.; Seguin, F.H.; Frenje, J.A.; Rinderknecht, H.G.; Sio, H.; Zylstra, A.B.; Petrasso, R.D.; Amendt, P.A.; Park, H.S.; Remington, B.A.; Ryutov, D.D.; Wilks, S.C.; Betti, R.; Frank, A.; Hu, S.X.; Sangster, T.C.; Hartigan, P.; Drake, R.P.; Kuranz, C.C.; Lebedev, S.V.; Woolsey, N.C.
    The remarkable discovery by the Chandra X-ray observatory that the Crab nebulaﾒs jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet.
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    Simulating radiative magnetohydrodynamical flows with ASTROBEAR: implementation and applications of non-equilibrium cooling
    (Oxford University Press, 2018) Hansen, E.C.; Hartigan, P.; Frank, A.; Wright, A.; Raymond, J.C.
    Radiative cooling plays a crucial role in the dynamics of many astrophysical flows, and is particularly important in the dense shocked gas within Herbig-Haro (HH) objects and stellar jets. Simulating cooling processes accurately is necessary to compare numerical simulations with existing and planned observations of HH objects, such as those from the Hubble Space Telescope and the James Webb Space Telescope. In this paper, we discuss a new, non-equilibrium cooling scheme we have implemented into the three-dimensional magnetohydrodynamic (MHD) code ASTROBEAR. The new cooling function includes ionization, recombination, and excitation of all the important atomic species that cool below 10 000 K. We tested the routine by comparing its predictions with those from the well-tested one-dimensional Cox–Raymond shock code (Raymond 1979). The results show that ASTROBEAR accurately tracks the ionization fraction, temperature, and other MHD variables for all low-velocity (≲90 km s−1) magnetized radiative shock waves. The new routine allows us to predict synthetic emission maps in all the bright forbidden and permitted lines observed in stellar jets, including H α, [N II], [O I], and [S II]. We present an example as to how these synthetic maps facilitate a direct comparison with narrowband images of HH objects.