Interstellar neutral oxygen in a two-shock heliosphere

A fraction of the interstellar neutral atoms crosses the interface between the solar wind and the ionized component of the local interstellar medium, and enters the heliosphere. The number of penetrating atoms depends on both the interface structure and the strength of the atom-plasma interaction, i.e. on the atom type. In this paper we consider the penetration of the interstellar oxygen. Oxygen atoms are of particular interest, because they are strongly coupled with the protons through charge exchange reactions, and because they are now observed under the form of pick-up ions and anomalous cosmic rays, providing new observational tests of the processes at work at the boundary of the heliosphere. Our calculations are based on the Baranov two-shock heliosphericmodel (Baranov andMalama, 1993). Oxygen number density distributions are computed through a Monte-Carlo simulation of the O and O+ flows through the H and H+ interface. The model accounts for direct charge-exchange (O atoms with protons), reverse charge-exchange (O ions with neutral H), photoionization and solar gravitation. Source distributions of O+ pick-up ions resulting from oxygen atom ionization are also produced, as well as density distributions of the oxygen ions in the heliosphere, assuming that newly created oxygen ions are instantaneously assimilated to the plasma. In the light of these results we compare the recent measurements of pick-up oxygen ions with the Local Interstellar Cloud oxygen abundance deduced from nearby star spectroscopy. The comparison shows that the measurements do not preclude, and even suggest, a significant ionization of the ambient interstellar medium.