1997A&A...326.1117N

We present a detailed spectroscopic analysis of the H and Hei spectrum of the Luminous Blue Variable (LBV) P Cygni. The observational constraints for our investigation are given by the mean optical and near-IR spectra obtained by Stahl et al. (1993A&AS...99..167S) and published UV, optical, IR and radio continuum measurements of the star. Within the parameter domain of interest we have investigated how line strengths, line shapes and the emergent energy distribution behave as a function of stellar parameters such as temperature, luminosity, wind density, etc. The sensitivity of the H and Hei line profiles to model parameters allows a quantitative spectroscopic analysis of P Cygni to be performed. The derived parameters for P Cygni place it near a regime where two classes of models exist - those in which H remains fully ionized in the wind, and those in which H eventually recombines in the wind. The transition between the two classes of models is dramatic - a 5% change in luminosity or mass-loss is sufficient. The shape of the P Cygni absorption on the Hα profile and the radio variability indicates that H must recombine in P Cygni's wind. As for Wolf-Rayet (W-R) stars we find that we can generate a set of homologous models which are capable of explaining, within observational errors, the flux distribution and the line profiles equally well. Scaling relations for the homologous models have been determined, but to first order they have the same T_eff and the same wind density parameter ({dot}(M)/R_*^1.5). Consequently it is impossible to derive R_* from a spectroscopic analysis. Assuming R_*=75R_☉, the following stellar parameters for P Cygni are derived: L_*=5.6x10⁵L_☉; (T_eff=1.82kK); n_He/n_H=0.3; {dot}(M)=3.0x10^–5M_☉/yr; V_∞=185km/s; β=2.5 and log(g_eff)=1.20. These imply a stellar distance of 1.7±0.1kpc, which is consistent with determinations in the literature. Continuum fluxes and the observed optical and near-infrared H and Hei line profiles are well reproduced by the model. The derived stellar parameters and the high helium abundance indicates that P Cygni is highly evolved.