RADIATION-DRIVEN WINDS OF HOT LUMINOUS STARS .8. A 1ST STEP TOWARDS DETAILED UV-LINE DIAGNOSTICS OF O-STARS

Improved radiation driven wind models are constructed to calculate det ailed synthetic UV spectra of hot luminous stars. The model improvemen ts comprise: a very detailed multilevel NLTE treatment of 29 of the mo st important ionization stages, in particular C, N, O and Fe using acc urate atomic data; the approximate inclusion of EUV radiation by shock heated matter in the ionization rates and a simple simulation of phot ospheric line blocking. Due to the improved NLTE opacities the calcula ted rates of mass-loss are increased by nearly a factor of two relativ e to previous models. The direct ionization by EUV shock radiation has important effects on the lines of the highest ionization stages, wher eas line blocking is needed to reproduce the lowest stages of ionizati on that are observed. A detailed comparison between observed and calcu lated synthetic spectra of two 0-stars, zeta Puppis in the Galaxy and Melnick 42 in the LMC, has been carried out to develop diagnostic tool s for the determination of stellar parameters, abundances and stellar wind properties from the ultraviolet spectra. The following results ar e obtained: Zeta Puppis. (i) The effective temperature obtained from t he analysis of optical hydrogen and helium absorption lines is confirm ed, however, the gravity is larger by 0.1 dex. (ii) A solar abundance of iron yields a reasonable fit of the rich Fe IV and V spectra. The a bundances of carbon, nitrogen and oxygen indicate that the atmosphere is mixed with CNO-burned material. (iii) A mass-loss rate of 5.1 10(-6 ) M. yr-1 resulting from a self-consistent wind model gives the best f it to the UV-spectrum. In this model helium recombines from He++ to He + in the outer wind layers as a result of the high wind density and ph otospheric line blocking in the wavelength region of the He II resonan ce line. Melnick 42. (i) The HST UV spectrum indicates a higher effect ive temperature than found by Heap et al. (1998) from the analysis of the optical lines. The stellar parameters that lead to an optimum fit of the UV-spectrum are: T(eff) = 50 500 K, log g = 3.785 (cgs), R/R. = 26, M/M. = 150, log(L/L. ) = 6.6. Melnick 42 is thus the most massive and most luminous star known. (ii) The mass-loss rate is 3 10(-5) M. yr-1 with and uncertainty of 50%. This value is in agreement with the observed emission strength of Halpha and He II 4686. (iii) The differe nce in T(eff) relative to Heap et al. (1991) is attributed to the effe ct of the strong stellar wind and its influence on the formation of th e optical hydrogen and helium lines. (iv) The abundance of all element s (including C, N, 0 and Fe) is reduced relative to the Sun by a facto r of four.