The orientation of the η Carinae binary system

Abstract

We examine a variety of observations that shed light on the orientation of the semimajor axis of the η Carinae massive binary system. Under several assumptions, we study the following observations: the Doppler shifts of some He i P Cygni lines that are attributed to the secondary's wind, of one Fe ii line that is attributed to the primary's wind and of the Paschen emission lines which are attributed to the shocked primary's wind, are computed in our model and are compared with observations. We compute the hydrogen column density towards the binary system in our model, and find a good agreement with that deduced from X-ray observations. We calculate the ionization of the surrounding gas blobs by the radiation of the hotter secondary star, and compare with observations of a highly excited [Ar III] narrow line. We find that all of these support an orientation where for most of the time the secondary - the hotter, less-massive star - is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage in its highly eccentric (e ≃ 0.9) orbit. Further supporting arguments are also listed, followed by a discussion on some open and complicated issues. © 2008 The Authors.