The stellar wind geometry of η Carinae

The Homunculus reflection nebula around eta Carinae provides a rare opportunity to observe a star from more than one direction. For eta Car, the nebula's geometry is known well enough to infer how wind profiles vary with latitude. STIS spectra of the Homunculus show directly, that eta Car has an aspherical stellar wind. P Cygni absorption in Balmer lines depends on latitude, with relatively high velocities and strong absorption near the polar axis. Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, and offer clues to eta Car's circumstellar structure. The bipolar wind geometry may imply that intrinsically asymmetric ejection helped form the Homunculus. It is also critical for understanding this star's variability and evolution.