The Binarity of Eta Carinae Revealed from Photoionization Modeling of   the Spectral Variability of the Weigelt Blobs B and D
  We focus on two Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) spectra of the Weigelt Blobs B&D, extending from 1640 to 10400A; one recorded during the 1998 minimum (March 1998) and the other recorded in February 1999, early in the following broad maximum. The spatially-resolved spectra suggest two distinct ionization regions. One structure is the permanently low ionization cores of the Weigelt Blobs, B&D, located several hundred AU from the ionizing source. Their spectra are dominated by emission from H I, [N II], Fe II, [Fe II], Ni II, [Ni II], Cr II and Ti II. The second region, relatively diffuse in character and located between the ionizing source and the Weigelt Blobs, is more highly ionized with emission from [FeIII], [Fe IV], N III], [Ne III], [Ar III], [Si III], [S III] and He I. Through photoionization modeling, we find that the radiation field from the more massive B-star companion supports the low ionization structure throughout the 5.54 year period. The radiation field of an evolved O-star is required to produce the higher ionization emission seen across the broad maximum. This emission region is identified with slow-moving condensations photoionized by the O star and located in the extended mass flow emanating from the B star primary. Comparison between the models and observations reveals that the high ionization region is physically distinct (n_H ~ 10^7 cm^(-3) and T_e ~ 10^4K) from the BD Blobs (n_H ~ 10^6 cm^(-3), T_e ~ 7000K). 
E. Verner, F. Bruhweiler (CUA and GSFC/NASA), and T. Gull (GSFC/NASA)