Carbon Abundance in WC Stars

Abstract

A specific feature of WC stars is the presence in their spectra of strong subordinate series lines of carbon ions arising in transitions between comparatively high-lying states. The intensities of these lines suggest that carbon abundance must be comparatively high in WC star envelopes because subordinate series lines are already nearly transparent to radiation (escape probability coefficients β_ik are close to unity) and their intensities strongly depend on the abundance of C(J_ki ∝ N_k). The resonance lines of carbon ions and the lines arising in the permitted transitions between the low-lying energy levels are in a wide abundance interval not very sensitive to abundance due to the fact that β_ik ≪ 1 for these transitions (J_ki ∝ N_k/N_i). For 3 WC stars (HD 165763 (WC5), HD 192103 (WC8) and HD 164270 (WC9)) we carried out the same kind of model-fitting study as for the WN stars (reported earlier). We used the same empirical models, that is we assumed that also for them the wind reaches the terminal velocity at R ≃ 2 R_o (v = const for R ≥ 2 R_o) and that T_e(R ≥ 2 R_o) = T ^II_e = 0.6 T_*, where T_* denotes the core temperature. We took into account the ions (atoms) of He, H and C for determining the ionization structure in the envelope.