NON-LTE SPECTRAL ANALYSES OF WOLF-RAYET STARS - THE NITROGEN SPECTRUMOF THE WN6-PROTOTYPE HD-192163 (WR136)

Non-LTE model calculations for Wolf-Rayet spectra of the nitrogen sequ ence (WN) are presented, accounting for a complex nitrogen model atom. A detailed fit to the observed spectrum of WR 136 alias HD 192163 (su btype WN6) is performed, providing an observational test of the ''stan dard model'' which implies the idealizing assumptions of stationarity, homogeneity and spherical symmetry. From the achieved agreement we co nclude that the ''standard model'' is basically adequate to describe t he atmosphere of WR 136 and thus, presumably, of other WN stars as wel l. Minor differences between the synthetic and the observed spectrum c an be interpreted as the signature of inhomogeneities (''clumping'') i n the stellar wind. Problems with individual nitrogen lines are attrib uted to deficiencies in the atomic data. The helium spectrum is hardly affected by the presence of nitrogen. This result justifies a posteri ori the application of pure-helium models for our many previous analys es of WN spectra. The parameters of WR 136 are determined as T = 55 k K, R = 6.4 R., M = 10(-3.84) M./yr, upsilon(infinity) = 1700 km/s, im plying a luminosity L = 10(5.5) L.. The quoted effective temperature T refers to the radius R. defined by a Rosseland optical depth of 30. When referring to tau(Ross) = 2/3, the effective temperature is only 2 8 kK because this atmosphere is spherically extended. The atmosphere o f WR 136 is composed of helium with 12% hydrogen and about 1.5% nitrog en (mass fractions). This hydrogen abundance is intermediate between t he evolutionary ''WNL'' and ''WNE'' stage, while standard calculations for post-red-supergiant evolution of massive stars predict a very sud den transition from higher hydrogen abundances to zero. The nitrogen a bundance is in accordance with the expectation for CNO-burned material .