η Carinae across the 2003.5 Minimum: Spectroscopic Evidence for Massive Binary Interactions

Abstract

We have analyzed high spatial, moderate spectral resolution observations of η Carinae (η Car) obtained with the STIS from 1998.0 to 2004.3. The data were obtained at discrete times covering an entire 2024 day spectroscopic cycle, with focus on the X-ray/ionization low state that began in 2003 June. The spectra show prominent P Cygni lines in H I, Fe II, and He I, which are complicated by blends and contamination by nebular emission and absorption. All lines show phase- and species-dependent variations in emission and absorption. For most of the cycle the He I emission is blueshifted relative to the H I and Fe II P Cygni emission lines, which are centered at approximately system velocity. The blueshifted He I absorption components vary in intensity and velocity throughout the 2024 day period. We construct radial velocity curves for the absorption component of the He I and H I lines. The He I absorption shows significant radial velocity variations throughout the cycle, with a rapid change of over 200 km s^-1 near the 2003.5 event. The H I velocity curve is similar to that of the He I absorption, although offset in phase and reduced in amplitude. We interpret the complex line profile variations in He I, H I, and Fe II to be a consequence of the dynamic interaction of the dense wind of η Car A with the less dense, faster wind plus the radiation field of a hot companion star, η Car B. We use the variations seen in He I and the other P Cygni lines to constrain the geometry of the orbit and the character of η Car B.