Detection of the Compressed Primary Stellar Wind in eta Carinae

A series of three Hubble Space Telescope/Space Telescope Imaging Spectrograph spectroscopic mappings, spaced approximately one year apart, reveal three partial arcs in [Fe II] and [Ni II] emissions moving outward from {eta} Carinae. We identify these arcs with the shell-like structures, seen in the three-dimensional hydrodynamical simulations, formed by compression of the primary wind by the secondary wind during periastron passages.