Modeling of the spectrum of Cygnus OB2 No. 7 supergiant

Abstract

We report the results of modeling of the spectrum of the O3 If* Cyg OB2 No. 7 supergiant in a broad wavelength range. We determine the physical properties and chemical composition of its atmosphere not assuming the presence of local thermodynamic equilibrium. The atmosphere reveals an excess of nitrogen X(N)/X(N)⊙ = 3.2 and the carbon and oxygen deficiency X(C)/X(C)⊙ = 0.08, X(O)/X(O)⊙ = 0.09. The lines in the stellar spectrum are divided into three groups which fail to be describedwithin a single model. Themodels describing each of these groups differ by themass-loss rate and the law of wind velocity variation. Thus, the numerical modeling suggests that the wind of the supergiant is heterogeneous. In addition, this paper describes the features of the CMFGEN code used and investigates the sensitivity of its results to the variations of different parameters.