On the evolution of Hα/N II in supernova remnants.

The line ratio Hα/N II in supernova remnants has been observed to evolve as a function of the expansion velocity. This is explained by assuming that the abundance of nitrogen with respect to hydrogen in the mass ejected by the supernova explosion or, prior to it, by the presupernova star, is higher than the average ambient value. The line ratio will increase as the relative abundance of nitrogen evolves towards the normal cosmic value as a result of accretion of interstellar material by the shock wave.