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I am not sure that I can answer your question, Miguel, as I am not an astrophysicist, but here are some pertinent facts. I hope some of our resident "physics heavies" will contribute to this thread.
Much to my chagrin, though I have spent considerable time studying the Interstellar Medium, I am still somewhat in the dark about the physical conditions in Supernova Remnants (SNRs)! Supernovae have enormous effects on the interstellar medium in a galaxy, and the consequent energy injection can either enhance or suppress star formation, depending upon the circumstances. Further, supernovae and SNRs pump lots of heavy elements into space, providing the raw materials for the formation of planets.
Lawrence H.Aller, in his classic , extremely detailed, but very clear & cogent, semi-popular-level book about stars and the interstellar medium, “Atoms, Stars and Nebulae”(3rd edn)(1991)(Cambridge University Press), writes that strong shock waves in the Interstellar Medium produce spectral lines of highly ionized atoms. In the optical regime, this implies the production of [OI], [OII], [OIII], and [SII] spectral lines. (also, plenty of UV lines are produced)
In a supernova remnant, a compressed shell or sheet of gas travels into a rarified gas with a speed exceeding that of sound in that medium, and this produces a shock wave. To quote Aller: The atoms in the advancing shock front are all moving parallel to one another, but the collision causes kinetic energy to be converted to heat”. Aller compares the collision between the surrounding diffuse interstellar medium and the gas expelled from a supernova or planetary nebula , to a bullet hitting a target and melting due to the conversion of kinetic energy into heat. There has to be a sudden rise in gas temperature and density where the shock wave hits the surrounding low-density gaseous medium.
The shell structures of young SNRs are known to expand at very high velocity, of the order 6000 km/s. According to the book ‘An Introduction to Radio Astronomy” by Bernard Burke and Francis Graham-Smith (a valuable and clear book that explains much about emission mechanisms in the interstellar medium), the shapes of the “shell-like” SNRs such as IC 443 and Gum Nebula and the Cygnus Loop are largely determined by the collision of the remnant with interstellar clouds.
Typical sizes of SNRs: The Crab Nebula is about 4 parsecs across, as is Cassiopeia A. The remnant of Tycho’s Supernova (1572) is about 9 parsecs across. The Cygnus Loop, a much older supernova remnant, is over 40 parsecs across. It is obvious, from the variety of sizes, that the physical conditions can vary very greatly between various Supernova Remnants, so it is going to be hard to say something that applies to the spectra of all SNRs.
C.R. Kitchin, in his book “Stars, Nebulae and The Interstellar Medium”(1987)(a useful simplification of astrophysics for non-physicists such as myself!), gives the following phases in the evolution of a SNR:
(1) Initial phase, where the density of the hot gases exploding outwards at about 10,000 km/s is very much higher than that of the surrounding interstellar medium, so that the expansion is essentially into a vacuum.
(2) When the SNR reaches about one parsec in size, the interstellar gas swept up by the expanding remnant reaches densities at which it starts to impede the expansion. This leads to the formation of a turbulent and strong shock wave, with strong synchrotron emission.
(3) The expansion velocity slowly decreases, and there is increasing emission of optical lines from heavier elements.
(4) The remnant will eventually merge with the interstellar medium, after a few hundred thousand years.
Best Regards,
mad galaxy man
TO BE CONTINUED, in another post!!!
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