A SPECTROSCOPIC INVESTIGATION OF P-CYGNI .1. H AND HE-I LINES

We present a detailed spectroscopic analysis of the H and He I spectru m of the Luminous Blue Variable (LBV) P Cygni. The observational const raints for our investigation are given by the mean optical and near-IR spectra obtained by Stahl et al. (1993) and published UV, optical, IR and radio continuum measurements of the star. Within the parameter do main of interest we have investigated how line strengths, line shapes and the emergent energy distribution behave as a function of stellar p arameters such as temperature, luminosity, wind density, etc. The sens itivity of the H and He I line profiles to model parameters allows a q uantitative spectroscopic analysis of P Cygni to be performed. The der ived parameters for P Cygni place it near a regime where two classes o f models exist - those in which H remains fully ionized in the wind, a nd those in which H eventually recombines in the wind. The transition between the two classes of models is dramatic - a 5% change in luminos ity or mass-loss is sufficient. The shape of the P Cygni absorption on the H alpha profile and the radio variability indicates that H must r ecombine 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 expla ining, within observational errors, the flux distribution and the line profiles equally well. Scaling relations for the homologous models ha ve been determined, but to first order they have the same T-eff and th e same wind density parameter ((M) over dot/R(1.5)). Consequently it is impossible to derive R from a spectroscopic analysis. Assuming R*= 75 R., the following stellar parameters for P Cygni are derived: [GRAP HICS] These imply a stellar distance of 1.7 +/- 0.1 kpc, which is cons istent with determinations in the literature. Continuum fluxes and the observed optical and near-infrared H and He I line profiles are well reproduced by the model. The derived stellar parameters and the high h elium abundance indicates that P Cygni is highly evolved.