An analysis of HST UV spectra of Cyg OB2 stars

Details An analysis of HST UV spectra of Cyg OB2 stars An analysis of HST UV spectra of Cyg OB2 stars

Feb 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001a%26a...366..623h&link_type=abstract

Astronomy and Astrophysics, v.366, p.623-635 (2001)

Astronomy and Astrophysics

Astrophysics

14

Stars: Atmospheres, Stars: Early-Types, Stars: Supergiants, Stars: Fundamental Parameters, Stars: Winds, Outflows, Galaxy: Open Clusters And Associations: Individual: Cyg Ob2

Scientific paper

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 v_infty . The terminal velocities agree well with the average v_infty vs. spectral class relationship compiled by Kudritzki & Puls (\cite{kp00}). We compare the observed v_infty 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 beta 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 T_eff 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, T_eff and : we find that N V increases with T_eff, Si Iv decreases and C Iv does not clearly correlate. As a byproduct, we also derive interestellar H I column densities towards Cyg OB2, which turn out to be quite large. For one object (Cyg OB2#2) we find inconsistencies making the association membership questionable. Partly based on INES data from the IUE satellite.

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