The Variable 6307 Å Emission Line in the Spectrum of Eta Carinae: Blueshifted [S III] λ6313 from the Interacting Winds

Abstract

The 6307 Å emission line in the spectrum of η Car, found by Martin et al., is blue-shifted [S iii] λ6313 emission originating from the outer wind structures of the massive binary system. We realized the identification while analyzing multiple forbidden emission lines not normally seen in the spectra of massive stars. The high spatial and moderate spectral resolutions of HST/STIS resolve forbidden lines of Fe⁺, N⁺, Fe⁺², S⁺², Ne⁺², and Ar⁺² into spatially and velocity-resolved ropelike features originating from collisionally-excited ions photoionized by UV photons or collisions. While the [Fe ii] emission extends across a velocity range of ± 500 km s^-1 out to 0.7″, more highly ionized forbidden emissions ([N ii], [Fe iii], [S iii], [Ar iii], and [Ne iii]) range in velocity from -500 to +200 km s^-1, but spatially extend out to only 0.4″. The [Fe ii] defines the outer regions of the massive primary wind. The [N ii], [Fe iii] emissions define the the outer wind interaction regions directly photoionized by far-UV radiation. Variations in emission of [S iii] λλ9533, 9071, and 6313 suggest density ranges of 10⁶ - 10¹⁰ cm^-3 for electron temperatures ranging from 8000 to 13,000 K. Mapping the temporal changes of the emission structure at critical phases of the 5.54 yr period will provide important diagnostics of the interacting winds.