In order to study the distribution of gas and ionizing radiation around η 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 Hubble Space Telescope/Space Telescope Imaging Spectrograph instrument during 1998-2004. Our principal results, some of them unexpected, are as follows. (1) The high-excitation fluxes varied systematically and non-trivially throughout η 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, blueshifted 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 eff ≈ 39, 000 K and L ~ 4 × 105 L , for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested. This research was supported by grant nos. GO-9973, 10844, and 11291 from the Space Telescope Science Institute. The HST is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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Published in The Astrophysical Journal, v. 710, no. 1, p. 729-742.

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