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Tomographic Separation of Composite Spectra. XI. The Physical Properties of the Massive Close Binary HD 100213 (TU Muscae)

We present the results of a Doppler tomographic reconstruction of the UV spectra of the double-lined, O binary HD 100213 based on observations made with the International Ultraviolet Explorer (IUE). We used cross-correlation methods to obtain radial velocities, confirm the orbital elements, estimate the UV flux ratio, and determine projected rotational velocities. The individual component spectra are classified as O7 V + O8 V using UV criteria defined by Penny, Gies, & Bagnuolo. We present a model fit of the eclipsing light curve from observations from the Hipparcos satellite and published observations of Andersen & Gronbech. We derive an orbital inclination, i=77.7deg+/-1.0deg. This analysis indicates that both stars are currently experiencing Roche lobe overflow (RLOF), which confirms earlier results that this is one of only a few massive contact binaries. Our derived masses, Mp/Msolar=16.7+/-0.4 and Ms/Msolar=10.4+/-0.4, are significantly lower than those computed from the Doppler shifts of lines in the optical spectrum. We suggest that the difference occurs because mutual irradiation decreases the upper atmospheric temperature gradient in the inward-facing hemispheres of both stars, which makes lower excitation lines appear weaker there and shifts their center of light away from the center of mass. We compare the current state of HD 100213 with predicted outcomes of massive close binary evolutionary models, and suggest that the system is currently in a very slow case AA mass transfer stage.