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

Abstract

We have analyzed high spatial, moderate spectral resolution observations of\nEta Carinae obtained with the STIS from 1998.0 to 2004.3. The spectra show\nprominent P-Cygni lines in H I, Fe II and He I which are complicated by blends\nand contamination by nebular emission and absorption along the line-of-sight\ntoward the observer. All lines show phase and species dependent variations in\nemission and absorption. For most of the cycle the He I emission is blueshifted\nrelative to the H I and Fe II P-Cygni emission lines, which are approximately\ncentered at system velocity. The blueshifted He I absorption varies in\nintensity and velocity throughout the 2024 day period. We construct radial\nvelocity curves for the absorption component of the He I and H I lines. The He\nI absorption shows significant radial velocity variations throughout the cycle,\nwith a rapid change of over 200 km/s near the 2003.5 event. The H I velocity\ncurve is similar to that of the He I absorption, though offset in phase and\nreduced in amplitude. We interpret the complex line profile variations in He I,\nH I and Fe II to be a consequence of the dynamic interaction of the dense wind\nof Eta Car A with the less dense, faster wind plus the radiation field of a hot\ncompanion star, Eta Car B. During most of the orbit, Eta Car B and the He+\nrecombination zone are on the near side of Eta Car A, producing blueshifted He\nI emission. He I absorption is formed in the part of the He+ zone that\nintersects the line-of-sight toward Eta Car. We use the variations seen in He I\nand the other P-Cygni lines to constrain the geometry of the orbit and the\ncharacter of Eta Car B.\n