The Absorption Spectrum of High-Density Stellar Ejecta in the Line-of-Sight to Eta Carinae

Abstract

Using the high dispersion NUV mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) to observe Eta Carinae, we have resolved and identified over 500 sharp, circumstellar absorption lines of iron-group singly-ionized and neutral elements with ~20 velocity components ranging from -146 km/s to -585 km/s. These lines are from transitions originating from ground and metastable levels as high as 40,000 cm-1 above ground. The absorbing material is located either in dense inhomogeneities in the stellar wind, the warm circumstellar gas immediately in the vicinity of Eta Carinae, or within the cooler foreground lobe of the Homunculus. We have used classical curve-of-growth analysis to derive atomic level populations for FeII at -146 km/s and for TiII at -513 km/s. These populations, plus photoionization and statistical equilibrium modeling, provide electron temperatures, Te, densities, n, and constraints on distances from the stellar source, r. For the -146 km/s component, we derive Te = 6400 K, n(H)>10e7 - 10e8 cm-3, and d ~1300 AU. For the -513 km/s component, we find a much cooler temperature, Te= 760 K, with n(H)> 10e7cm-3, we estimate d~10,000 AU. The large distances for these two components place the absorptions in the vicinity of identifiable ejecta from historical events, not near or in the dense wind of Eta Carinae. Further analysis, in parallel with obtaining improved experimental and theoretical atomic data, is underway to determine what physical mechanisms and elemental abundances can explain the large number of strong circumstellar absorption features in the spectrum of Eta Carinae.