Investigation of CygOB2 #11 (O5 Ifc) by modeling its atmosphere
  We continue the study of O-supergiants belonging to the association Cyg OB2 using moderate-resolution spectra. In this paper we present results of the modeling of the stellar atmosphere of Cyg OB2 #11. This object belongs to the spectral class Ofc, which was recently introduced and yet small in numbers. Ofc class consists of stars with normal spectra with CIII {\lambda}{\lambda}4647,4650,4652 emission lines of comparable intensity to those of the Of-defining lines NIII {\lambda}{\lambda}4634,4640,4642. We combined new spectral data obtained by the 1.5-m Russian-Turkish telescope with spectra from MAST and CASU archives and determined physical parameters of the wind and chemical composition of the stellar atmosphere using CMFGEN code. The estimated nitrogen abundance is lower than one in atmospheres of normal O-supergiants (i.e. O4-6 supergiants without additional spectral index "n" or "c") and carbon abundance is solar. Also we find an excess in silicon. We present an illustrative comparison of our modeling results with current Geneva evolutionary models for rotating massive stars. The position on the Hertzsprung-Russell diagram corresponds to the star mass of about 50M\sun and age about 4.5 Myr. Moreover, we carried out the high angular resolution (about 0.02'') observations on the Russian 6-meter telescope aiming to find weaker companions of this star, which did not reveal any. 
Olga Maryeva, Roman Zhuchkov and Eugene Malogolovets
astro-ph.SR