Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...272..571s&link_type=abstract
Astronomy and Astrophysics, Vol. 272, p. 571 (1993)
Astronomy and Astrophysics
Astrophysics
2
Hydromagnetics, Shock Waves, Supernova Remnants: General
Scientific paper
The degree of compression in radiative shocks (discontinuous jump followed by strong cooling) is crucial to the distribution of the phases of interstellar gas. The influence of the magnetic field, especially its direction, is studied here. In fact plane-parallel radiative shocks are switch type for a wide range of conditions. In such shocks, the magnetic field is turned through a large angle from being quasi-parallel. It is shown here that plane-parallel isothermal shocks are switch type for magnetic fields within an angle 1/A of the shock normal where A is the Alfvén number, with the stringent condition that the Alfvén speed exceeds the sound speed. The switch is usually located within the dense cooling layer rather than the shock front. Super-cooling, in which the downstream temperature drops below the pre-shock value, is often encountered. A super-cooling layer can be switch type even if the Alfvén speed is much less than the sound speed. The degree of compression is then still limited by the magnetic pressure rather than thermal pressure.
Applications and predictions pertaining mainly to the interstellar medium are presented. Switch off and intermediate shocks are also studied. Supernova and super-bubble shell structures may then provide evidence for interstellar gas properties. The switch-off shock can be recognised by synchrotron radiation signatures. Also the switch-off temperature jump is shown to be closely related to the value of the ratio of Alfvén to sound speeds. Super-cooled intermediate shocks, existing when the plane-parallel assumption is relaxed, can also occur under an extended range of pre-shock states. Hence the "dimple" shell structures of some remnants do not provide evidence for high Alfvén speeds. Finally, torsional switches are presented as an alternative to intermediate waves in interpreting shell structures.
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