IONIZATION STRUCTURE OF CYGNUS-X-3 - IRON ANOMALY IN WOLF-RAYET STARS

Ionization structure of the Cygnus X-3 binary system is simulated on t he basis of the stellar wind model of the luminosity modulation and th e X-ray spectra observed with GINGA satellite. Emergent spectra are co mpared with observations to derive constraints on the companion mass a nd metallicity of the stellar wind. The iron edge absorption energy su ggests that the companion is fairly massive; M = 7(-2)(+3), M.. Combin ing the estimated companion mass with the column density of incomplete ly ionized iron atoms, we find that iron in the stellar wind is somewh at depleted compared with the cosmic abundance; 0.1 similar to 0.5 tim es the cosmic abundance. The phase-to-phase spectral variation observe d with GINGA is well reproduced if the stellar wind gas is enriched wi th either C, N, or O. These results strongly support the recent infrar ed observation that the companion of Cygnus X-3 has the feature charac teristic to N-rich Wolf-Rayet stars. The inferred iron depletion may b e suggestive to a binary evolution and the nature of Wolf-Rayet stars. A modest interpretation is that the companion of the Cygnus X-3 is cl assified to an early WN star (WNE) and the metal deficit is due to the galactic abundance gradient or the local abundance anomaly.