THE ASCA X-RAY-SPECTRUM OF ETA CARINAE

We have obtained high signal-to-noise ratio 0.5-10 keV band X-ray spec tra of the peculiar, extremely luminous star eta Carinae with the Adva nced Satellite for Cosmology and Astrophysics (ASCA) X-ray observatory during Cycle 4 observations in mid-1996. These data comprise the best X-ray spectra to date of the cool source (kT similar to 0.3 keV) surr ounding the homunculus and of the hot source (kT similar to 5 keV) ass ociated with eta Car itself. We identify line emission from ions of N, Mg, Si, and S and numerous strong transitions of Fe in a variety of i onization stages, including the first clear identification of a fluore scent Fe line produced by photoionization of cool material by the X-ra y continuum from the hot source. The line strengths are consistent wit h thermal equilibrium models, though abundances of some important elem ents are nonsolar. Our analysis suggests that Fe is slightly underabun dant and S and Si somewhat overabundant. Most importantly, we confirm the high N enrichment derived by Tsuboi et al. from their analysis of a shorter ASCA observation obtained during the performance verificatio n (PV) phase in 1993 August. The O/N abundance ratio derived from the ASCA spectra is consistent with an upper limit from optical/UV spectra l analysis (Davidson et al.) and with predictions of evolutionary mode ls for extremely massive stars. Comparison of the Cycle 4 and PV-phase spectra shows that the X-ray luminosity increased by similar to 50% d uring this 3 year interval. Using the ASCA spectral model as a templat e, we reevaluate the spectrum of II Car obtained by the Rontgen Sateli te (ROSAT) in late 1992 and construct an X-ray light curve for the 199 2.4-1996.6 interval. We present spectra from the Intel national Ultrav iolet Explorer (IUE) satellite obtained at nearly the same time as the Cycle 4 ASCA spectra and show that the observed X-ray variability is reflected in changes of some important UV spectral features. Our data suggest that the X-ray emission and state of the stellar wind are inti mately connected, though the exact mechanism of coupling is not known. We suggest two alternatives: an underlying photospheric change of und etermined origin in eta Car itself, or a collision between a dense ste llar wind from eta Car and the wind or photosphere of a companion.