INTERSTELLAR NEUTRAL OXYGEN IN A 2-SHOCK HELIOSPHERE

A fraction of the interstellar neutral atoms crosses the interface bet ween the solar wind and the ionized component of the local interstella r medium, and enters the heliosphere. The number of penetrating atoms depends on both the interface structure and the strength of the atom-p lasma interaction, i.e. on the atom type. In this paper we consider th e penetration of the interstellar oxygen. Oxygen atoms are of particul ar interest, because they are strongly coupled with the protons throug h charge exchange reactions, and because they are now observed under t he form of pick-up ions and anomalous cosmic rays, providing new obser vational tests of the processes at work at the boundary of the heliosp here. Our calculations are based on the Baranov two-shock heliospheric model (Baranov and Malama, 1993). Oxygen number density distributions are computed through a Monte-Carlo simulation of the O and O+ flows t hrough the H and H+ interface. The model accounts for direct charge-ex change (O atoms with protons), reverse charge-exchange (O ions with ne utral H), photoionization and solar, gravitation. Source distributions of O+ pick-up ions resulting from oxygen atom ionization are also pro duced, as well as density distributions of the oxygen ions in the heli osphere, assuming that newly created oxygen ions are instantaneously a ssimilated to the plasma. In the light of these results we compare the recent measurements of pick-up oxygen ions with the Local Interstella r Cloud oxygen abundance deduced from nearby star spectroscopy. The co mparison shows that the measurements do not preclude, and even suggest , a significant ionization of the ambient interstellar medium.