The Apparent Size of Rapidly Rotating Stars

The stellar interferometer at Narrabri Observatory measures the angular diameter of hot, bright stars. It does this by matching the observed intensity correlation from two detectors, as a function of separation between the detectors, with that from a uniform disk (see Hanbury Brown et al.). When this measurement is taken in conjunction with experimental determinations of the monochromatic flux received at the Earth, the effective temperature of the star, T_e, can be determined. However, if the star being observed is rotating rapidly, comparison with a uniform disk is of doubtful validity. Owing to its rotation the shape of the star will be distorted, and its effective temperature will vary over the surface (being apparently hotter at the poles). Therefore the measured angular diameter of the star will be different, and will change with the orientation of the star’s axis of rotation in the sky.