Line formation in the wind of alpha Cygni

We summarize the results of recent IUE observations of the spectra of α Cyg and discuss possible explanations for the Doppler-shifted deep absorption profiles in the resonance lines of the singly ionized metals. The results of detailed modeling for Mg II lambda2802 are presented and discussed in the context of observations of α Cyg in the radio, infrared, visible, ultraviolet, and X-ray parts of the spectrum. Extensive semiempirical modeling was carried out in which the Mg+ resonance transition was treated as a NLTE two-level atom in an expanding, spherically symmetric atmosphere. Supersonic microturbulence in a cool wind is required for reasonable agreement with observations. In such winds, the density and the photoionization rate, which is determined by radiation temperature T/sub r/ at the Mg+ ionization threshold, control the strength of the line and influence the shape of its profile. For reasonable deviations of T/sub r/ from Groth's value based on a LTE calculation, successful models were found with M ranging from 3.1 x 10-9 to 1.5 x 10-7 M/sub sun/ yr-1. Although a unique model cannot be determined by modeling the saturated lambda2802 profile in α Cyg, a qualitative explanation first proposed by Lamers is confirmed for this unusual line, which is characterized by Doppler-shifted broad saturation, a crisp violet edge, and the absence of emission or absorption longward of its interstellar absorption feature. As suggested by Lamers, photons removed by scattering from the shortward side of the line just fill in the absorption line formed in the photospheric region. Cold thermalizing winds and other alternative models are discussed, and evidence is presented for deceleration in the outermost layers of the flow