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, Wojtek Krzemiński, and published in the Astrophysical Journal (Krzemiński, 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 ellipsoidal 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.