Ultraviolet Spectral Morphology of the Stellar Core of eta Carinae

Abstract

We present high-quality spectra of eta Car between 1203 and 1765 A obtained with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A square entrance slit 0."22 in diameter was used to isolate the brightest region of the core, "component A," from the nearby knots of ejecta and the nebulosity in which it is embedded. The spectrum is that of an unusually dense stellar envelope, with a nearly flat continuum and numerous emission and absorption lines. Most lines exhibit the P Cygni type profiles characteristic of a stellar wind. Lines from a wide range of ionization stages, from N I through N V, are visible, most having very similar velocity structures. Wind lines from C, N, and O are all clearly visible, as are numerous metallic species. Most of the lines have two discrete, well-resolved absorption components, one centered near -500 km s-1 and the other near -1100 km s-1. The overall morphology does not correspond to any one normal spectral type but suggests a composite of features seen in B-type supergiants in the range B2 Ia to B8 Ia, with additional lower temperature lines also present. The spectrum of eta Car is qualitatively similar to that of P Cyg in this wavelength range.