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Shock weakening induced by radiation losses

Figure 1: Cell for radiative shock experiment, realized at GEPI/Observatoire de Paris

Experiments performed on PALS laser installation in december 2005 pointed out the importance of the radiation losses in the dynamics of the precursor. In these experiments, a shock wave is generated in a miniaturized shock tube ( picture) in xenon at 0.2 bar. The cell is parallelipedic and has an internal section of 0.7 x 0.7 mm. The shock wave was generated by the conversion of PALS laser (200 J at 438 nm in 0.3 ns) into a mechanical energy on a composite micronic scale piston, which propagates inside the tube at a velocity of about 60 km/s.



Figure 2: variation with time of the position of the foot of the radiative precursor versus time

Even in a geometry which seams to be 1D from a pure hydrodynamical point of view, the radiation may escape from the transparent walls of the shock tube. This leads to alteration of the shock wave , which departs from this 1D topology, with bending on the border of the cell, confirming previous results (Leygnac et al 2006). These experiments proved that this radiation losses lead also to a weakening of this wave (Gonzalez et al. 2006). This illustrates the importance of the boundary conditions for the radiative flux in the topology and dynamics of experimental radiative shocks.

Using HERACLES (Gonzalez et al. AA 464, 429, 2007) simulations, we studied the variation of the shock dynamics and also the asymptotic stationary limit by varying the fraction of the radiation which is reflected on the cell walls. We proved that these boudary conditions have a strong influence on the time which is needed to achieve the stationary limit. This important result indicates that it is possible the study of the stationary limit in the laboratory.

These last results are very important for astrophysical situations. Hence, as the time needed to achieved the stationary limit is - in general - far smaller than the traveling times of the shock wave in the medium, radiative shocks are often (but not always) stationary. This is the case, for instance in the case of the shock waves in pulsating stars like cepheids or Miras, and also in accretion shocks .

The question of radiation losses is very important for the description of the accretion shocks in classical T Tauri stars. The relevant parameter is there the ratio of the photon mean free path to the diameter of the accretion column, those topology is completely unknown.

References


"Astrophysical radiative shocks: from modelling to laboratory experiments", Gonzalez, M., Stehle, C. , Audit. E., Busquet, M., Rus, B., Thais, F., Acef, O., Barroso, P., Bar-Shalom, A., Bauduin, D., Kozlova, M., Lery, T., Madouri, A. , Mocek, T., Polan, J., Laser Particle Beams, vol 24, 535-540 (2006) Reprint

"Effect of lateral radiative losses on radiative shock propagation", M. Busquet, E. Audit, M. Gonzalez, C. Stehle, F. Thais, O. Acef, D. Bauduin, P. Barroso, B. Rus, M. Kozlova, J. Polan, T. Mocek, High Energy Density Physics, 3, 8-11 (2007), Reprint

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