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C-H Lyu, F.C. Bruhweiler
We present time-dependent ionization calculations in an attempt to explain the recent high He ionization fractions deduced from EUVE observations for gas in the local interstellar medium (LISM), especially in the Local Cloud in which the Sun is embedded. The observed high He ionization fractions and low upper limits for the local EUV ionizing radiation field for He imply nonequilibrium ionization for this element. In this paper, we explore what effects a recent supernova explosion, possibly linked to the origin of the nearby Loop I supernova remnant, might have on the observed ionization of both H and He in the LISM We include contributions from the local EUV radiation field, collisional ionization, radiative and dielectronic recombination, and charge exchange in our time-dependent code. Our results indicate that the observed He and H ionization likely denote gas in a nonequilibrium ionic recombination phase. These calculations clearly show that a UV flash associated with a supernova would produce only trace H and He ionization in the Local Cloud unless the ambient interstellar gas were within 20 pc of the explosion. More likely, time-dependent ionization at the Sun would be due to the resulting shocks produced by this event. If shocks are responsible for the observed He ionization, between 2 and 3.4 million yr have elapsed since the start of the H and He recombination phase. We find that the local EUV radiation from discrete stellar sources and the surrounding 106 K hot plasma still can have significant effects on the ionization of H and He, respectively, in the Local Cloud. More importantly, our time-dependent calculations can reproduce the H and He ionization fractions observed from EUVE The He ionization fraction seems to be a very sensitive diagnostic for time-dependent ionization in the LISM. However, further observations and additional constraints are necessary before deducing a unique history for the LISM.
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