2006ApJS..163..173G

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) high-resolution near-ultraviolet (NUV) spectra of η Car's central source recorded before, during and after the 2003.5 spectroscopic minimum. Our focus is on the narrow absorption lines formed in multiple circumstellar shells, superimposed on the broad P Cygni stellar wind features, and how they respond to the flux variations of η Car across the minimum. Over 30 separate narrow-line velocity components are identified: three around -146 km/s, many between -323 and -587 km/s, and a few exceeding -1000 km/s. In general, excitation decreases with increasing velocity indicating that the primary excitation is by UV stellar photons and that the slower shells are located closer to the central source. Two well-isolated velocity systems at -146 and -513 km/s display very different spectral characteristics regarding ionization/excitation and respond differently to η Car's spectroscopic minimum. The -146 km/s shell, associated with the (internal) Little Homunculus, is ionized across the broad spectroscopic maximum but relaxes during the few months long minimum. The -146 km/s component is joined by adjacent velocity components in lines of singly ionized iron-group elements. Ti II and V II absorptions appear during the minimum, most likely caused by a decrease in Lyman-ionizing flux. The -513 km/s component, part of the (outer) Homunculus, does not show significant changes in atomic absorptions, but intermediate-velocity components between -369 and -503 km/s vary during the minimum. We present the NUV spectrum and describe its general characteristics, but we focus on identifications of the nebular absorption lines and their variations across the 2003.5 minimum. The complete spectrum between 2380 and 3160 Å with line identifications is available in the electronic edition of this paper.