Relative ionizations in the nearest interstellar gas

We compare CLOUDY predictions for the equilibrium ionization in the interstellar cloud surrounding the solar system with pick-up ion data. The incident radiation field includes contributions from hot stars, the emission from the conductive cloud boundary and the diffuse far-UV background. To within the observational uncertainties, CLOUDY predictions for the ratios n(He)/n(O), n(N) /n(O), n(Ne)/n(O), and n(He)/n(Ne) are consistent with pick-up ion data, provided that O and N are filtered by about 50 percent in the heliopause region and the outer heliosphere as predicted by others. Thus, the steady-state ionization model and assumed radiation field appear approximately valid. However, the youth and low intervening column density towards the Vela pulsar leave open the possibility that the parent supernova explosion about 10,500 years ago, and 200 pc distant, may also have affected LISM ionization, although the mechanism is uncertain. Support for this last possibility is provided by the apparent signature of the Vela explosion in the terrestrial geological record. (Author)