Krzeminski's Star
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Krzeminski's Star is a 20.5 solar mass slightly evolved blue supergiant with a radius of 11.8 solar radii and spectral type O6.5II (see stellar classification). It is estimate to be 8 kpc distant. Most notably, it is the visible component of the occulting spectroscopic binary system that forms the X-ray pulsar called Cen X-3.
Despite detailed data from the Uhuru satellite as to the orbital period of the binary, and the pulsation period in the X-ray band as well as the minimum mass of the occulting star, the optical component remained undiscovered for three years.
This was partly because Cen X-3 lies in the plane of the Galaxy in the direction of the Carina Spiral Arm, and so observations were forced to differentiate among dozens of faint objects. Centaurus X-3 was finally identified with a faint, heavily reddened variable star lying just outside the error box predicted by Uhuru observations. The visible star was later named after its discoverer, and published in the Astrophysical Journal (Krzeminski, W 1974 Astrophys J. 184 271).
There is little doubt as to the correctness of the optical candidate, since it is in apparent agreement with the period and phase of Cen X-3, and exhibits the same similarity in its double wave and amplitude light curve seen in other known massive binary systems. The double wave ellipsidal light variations are produced by a tidally deformed giant that nearly fills its Roche lobe. Also, the visible component corresponds to a BO II class star, comparable with the mass derived from X-ray data, consistent with the minimum radius that has been fixed by X-ray eclipse duration.
Using a BO II stellar classification model, the distance to Cen X-3 is calculated at 8 kpc. A lower limit on the distance can be estimated from the relation between luminosity, effective temperature and minimum radius of the optical star. Assuming an effective temperature of 30,000 kelvins, for a BO type star, and a bolometric correction of -2.90, the lower limit for the distance is 6.2 kpc.
