Non-LTE spectral analyses of Wolf-Rayet stars: The nitrogen spectrum of the WN6 prototype HD 192163 (WR136)

Details Non-LTE spectral analyses of Wolf-Rayet stars: The nitrogen spectrum of the WN6 prototype HD 192163 (WR136) Non-LTE spectral analyses of Wolf-Rayet stars: The nitrogen spectrum of the WN6 prototype HD 192163 (WR136)

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...281..184h&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 281, no. 1, p. 184-198

Astronomy and Astrophysics

Astrophysics

23

Abundance, Line Spectra, Nitrogen, Spectrum Analysis, Stellar Atmospheres, Wolf-Rayet Stars, Chemical Composition, Doppler Effect, Emission Spectra, Helium, Radiative Transfer

Scientific paper

Non-Local Thermodynamic Equilibrium (LTE) model calculations for Wolf-Rayet spectra of the nitrogen sequence (WN) are presented, accounting for a complex nitrogen model atom. A detailed fit to the observed spectrum of WR 136 alias HD 192163 (subtype WN6) is performed, providing an observational test of the 'standard model' which implies the idealizing assumptions of stationarity, homogeneity and spherical symmetry. From the achieved agreement we conclude that the 'standard model' is basically adequate to describe the atmosphere of WR 136 and thus, presumably, of other WN stars as well. Minor differences between the synthetic and the observed spectrum can be interpreted as the signature of inhomogeneities ('clumping') in the stellar wind. Problems with individual nitrogen lines are attributed to deficiencies in the atomic data. The helium spectrum is hardly affected by the presence of nitrogen. This result justifies a posteriori the application of pure-helium models for our many previous analyses of WN spectra. The parameters of WR 136 are determined as Tstar = 55 kK, Rstar = 6.4 solar radii, dot-M = 10-3.84 solar mass/yr, vinfinity = 1700 km/s, implying a luminosity L = 105.5 solar luminosity. The quoted effective temperature Tstar refers to the radius Rstar defined by a Rosseland optical depth of 30. When referring to tauRoss = 2/3, the effective temperature is only 28 kK because this atmosphere is spherically extended. The atmosphere of WR 136 is composed of helium with 12% hydrogen and about 1.5% nitrogen (mass fractions). This hydrogen abundance is intermediate between the evolutionary 'WNL' and 'WNE' stage, while standard calculations for post-red-supergiant evolution of massive stars predict a very sudden transition from higher hydrogen abundances to zero. The nitrogen abundance is in accordance with the expectation for CNO-burned material.

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