V348 Sagittarii : analysis of the [WC12] stellar spectrum

Abstract

The Wolf-Rayet type spectrum of the highly variable V 348 Sgr, central star of a planetary nebula, is analyzed. An optical spectrum during maximum shows various emission and P-Cygni features, which indicate an expansion of the atmosphere. In order to assess the stellar and atmospheric parameters of this Population II object, a high resolution optical observation is compared with synthetic spectra from model calculations for cool and carbon-rich Population I Wolf-Rayet stars. The model atmosphere contains the elements hydrogen, helium and carbon. The observation is reproduced best with the model parameters T* = 20,000 K, R* = 6 solar radius, M-dot = 10^-6.5 solar mass/yr and V_infinity = 190 km/s. The modeled line strengths vary only with the ratio R*/M-dot^2/3, which is determined by this analysis, while the absolute fluxes scale with R²*. The mentioned model parameters imply M_V = -2.84 mag (Johnson system) or M_V = -2.94 mag (Stromgren system). The element abundances in particle fractions are epsilon_H = 0.20, epsilon_He = 0.55 and epsilon_C = 0.25. The presence of hydrogen in material which otherwise shows much carbon produced by helium burning leads to the conclusion that violent mixing must have happened during the evolution of that star. Since only C II emission features are visible in the observed spectrum - C III lines are not present - the spectroscopical classification of V 348 Sgr is (WC 12).