Tomographic Separation of Composite Spectra. XII. The Physical Properties and Spectral Phase Variability of the Massive Close Binary HD 159176

Abstract

We present our analysis of the double-lined, O-binary HD 159176 based on observations made with the International Ultraviolet Explorer (IUE). We use cross-correlation methods to obtain radial velocities, confirm the orbital elements, and estimate the UV flux ratio. In addition, we cross-correlate specific regions of the IUE spectra corresponding to Fe v, Fe IV, He II, N III, and O IV features to determine the phase dependence of the cross-correlation strength for those features. We tomographically reconstruct the individual component spectra, which we classify as O6.5 V + O7 V. We present the first light-curve analysis of observations from the HIPPARCOS satellite combined with data from Thomas & Pachoulakis and determine an orbital inclination, I=43\buildrel{\circ}\over{.} 5+/- 4\buildrel{\circ}\over{.} 5. Both stars are well within their Roche surfaces. Our derived masses, {M}_p/{M}_⊙ =46.4{+/- }_9.5^14.3 and {M}_s/{M}_⊙ =44.{+/- }_9.1^13.6, are constrained by the known distance and reddening to NGC 6383 and the derived optical flux ratio, and agree within errors with predicted evolutionary masses. Both components display phase variations corresponding to the classical Struve-Sahade effect in the UV, Fe IV, N III, and O IV cross-correlation functions. The Fe v features have remarkably uniform profile strengths except in observations prior to conjunctions, a possible post-quadrature Struve-Sahade effect. The derived orbital elements from the various ions are not affected by the Struve-Sahade variability. Both the spectral variability and derived orbital semi-amplitudes of He II λ 1640 suggest that this feature is formed preferentially on the inner hemispheres of the component stars.