HYDRODYNAMIC ATMOSPHERE MODELS FOR HOT LUMINOUS STARS

We present the first line blanketed hydrodynamic models of spherically expanding atmospheres of hot stars. The models are characterised by a simultaneous solution of the equation of motion, the non-LTE populati ons of hydrogen and helium, and radiation transfer in a line blanketed atmosphere. The entire domain from the optically thick photosphere ou t to the terminal velocity of the wind is treated. The radiative force s are evaluated consistently with the depth-dependent radiation field, taking into account multiple scattering by metal lines and line overl ap. This allows us to determine mass loss rates and the velocity field resp. density structure, as well as to predict the line blanketed ene rgy distribution, the photospheric absorption lines, and the emission features emerging from the wind. The major improvements over unified n on-LTE model atmospheres advocated by the Munich group (Gabler et al. 1989) are twofold: 1) The effects of line blanketing for the radiation transfer and statistical equilibrium of hydrogen and helium are inclu ded in the atmosphere calculations. 2) The radiative force (resp. line force parameters k, alpha) is evaluated using the depth-dependent rad iation field of the model atmosphere We present a detailed discussion of the influence of the photosphere-wind transition zone on line profi les and the effects of line blanketing on a hydrodynamic non-LTE model atmosphere. Two important results are obtained from our study: 1 .We quantify the influence of line blanketing on the atmospheric structure and on the predicted spectrum. In particular, we qualitatively confir m the results obtained with core-halo models and find that the correct ions of Abbott & Hummer (1985) and Bohannan et al. (1986, 1990) are al so quantitatively correct. 2. We show that even ''purely'' photospheri c lines, on which spectroscopic determinations of basic stellar parame ters rely, are strongly affected by the velocity field in the transiti on zone between the photosphere and the wind, and not only by the mass loss rate. Thus, for the more luminous OB stars spectroscopic analyse s not only depend on three parameters (g, T(eff), H/He abundance), but also on the atmospheric structure of the wind (i.e. M, upsilon(r)). T herefore, we add new evidence to the previously stated finding that fo r precise determinations of stellar parameters and abundances of hot l uminous stars, the use of plane parallel models may lead to systematic errors. This implies that the recent finding of discrepancies of spec troscopic masses and helium abundances compared to predictions of stan dard evolutionary models could be due to the inappropriateness of phot ospheric models for the analysis of luminous stars. The stellar parame ters of our models are those thought to be representative for the O4 I (n)f star zeta Puppis. A comparison of the synthetic spectra with the observations shows that our model fits are not satisfactory. We find g ood agreement only for the key lines of a spectroscopic analysis, i.e. Hgamma, HeI lambda4471, and HeII lambda4542. However, for all lines t hat show wind features our predictions are clearly not correct. Since a spectroscopic analysis is a multi-dimensional problem it is impossib le to single out one stellar parameter that is responsible for the fai lure of the model. We tentatively interpret our result as an indicatio n that the calculated wind structure is not correct. The reason is not obvious, but it could be simply that the commonly adopted distance to the star is wrong. In any case, the spectrum of zeta Puppis should be carefully reanalysed with hydrodynamic model atmospheres.