In order to study the distribution of gas and ionizing radiation around \eta Car and their implications for its likely companion star , we have examined high-excitation emission lines of [ Ne iii ] , [ Fe iii ] , etc. , in spectra obtained with the HST/STIS instrument during 1998–2004 .
Our principal results , some of them unexpected , are : ( 1 ) The high-excitation fluxes varied systematically and non-trivially throughout \eta Car ’ s 5.5-year spectroscopic cycle .
Instead of rising to a plateau after the 1998 “ event , ” they changed continuously with a maximum in mid-cycle .
( 2 ) At one significant location a brief , strong secondary maximum occurred just before the 2003.5 spectroscopic event .
( 3 ) These emission lines are strongly concentrated at the “ Weigelt knots ” several hundred AU northwest of the star .
With less certainty , [ Ne iii ] appears to be somewhat more concentrated than [ Fe iii ] .
( 4 ) A faster , blue-shifted component of each feature appears concentrated near the star and elongated perpendicular to the system ’ s bipolar axis .
This structure may be related to the equatorial outflow and/or to dense material known to exist along our line of sight to the star .
( 5 ) Using the photoionization program Cloudy , we estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the ejecta . T _ { \mathrm { eff } } \approx 39000 K and L \sim 4 \times 10 ^ { 5 } L _ { \odot } , for example , would be satisfactory .
The allowed region in parameter space is wider ( and mostly less luminous ) than some previous authors suggested .