An analysis of HST UV spectra of Cyg OB2 stars

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Abstract
As a first step of a vigorous program to investigate the Wind Momentum - Luminosity Relationship (WLR) of Galactic O-stars by analyzing stars belonging to the same cluster we present UV HST observations of six supergiants and one giant in the Cyg OB2 association. Terminal and turbulent wind velocities, velocity laws and metal ion column densities are derived and mean ionization fractions are estimated. Turbulent velocities are mostly in the range 10-14% of . The terminal velocities agree well with the average vs. spectral class relationship compiled by Kudritzki & Puls ([CITE]). We compare the observed vs. escape velocity (depending on the diagnostics of the stellar mass) correlation with the predictions of the radiatively driven wind theory and find better agreement with the spectroscopic masses rather than with the evolutionary ones. The β velocity field exponents are in the range 0.7-0.8, without any trend towards larger values. We show that for a single luminosity class there is a tight relationship between Teff and (the mean density at the point in the stellar wind, where half the wind terminal velocity is reached). In consequence, the ionization fractions show the same trend with both, Teff and : we find that Nv increases with Teff, Siiv decreases and Civ does not clearly correlate. As a byproduct, we also derive interestellar Hi column densities towards Cyg OB2, which turn out to be quite large. For one object (Cyg OB22) we find inconsistencies making the association membership questionable.