AN INSTABILITY IN THE RADIATIVE IONIZATION OF ATOMIC HYDROGEN HELIUM GAS/

We show that the process of photoionizing a gas of atomic hydrogen and helium by line radiation whose energy is slightly above the helium si ngle-ionization threshold is unstable if the helium fraction by number is less than approximately one-half. However, in the two scenarios we consider here, based on the decaying dark matter (DDM) model of cosmo logical reionization, there is no significant growth. In the first sce nario, we consider ionization and recombination to be approximately in equilibrium. This is relevant to high photon flux rates and early rei onization, but in that case the heating is balanced by Compton cooling , which is very stabilizing. In the second scenario, we ignore recombi nation. This is relevant to low photon flux rates, or to the last stag e of the reionization. In that case, there is too little growth on a c osmological timescale to be significant.