Determination of Aspect and Degree of Differential Rotation, From Line Profiles in Rapidly Rotating Stars

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Abstract
The work presented here is an attempt to demonstrate that the angles of inclination of the rotational axes, as well as the presence or absence of differential rotation, can be determined from accurate measurements of absorption line profiles in the spectra of rapidly rotating stars, using second-order effects in these profiles caused by departures of the figures of such stars from spherical symmetry. An important tool is the fact that the second-order effects differ for different absorption lines. Computations of rotationally broadened profiles of He I 4471, Mg II 4481, and Ca II 3934 are presented and compared with measurements in five bright, rapidly rotating B and A type main sequence stars. Preliminary conclusions are that the surface rotation law appears to be either solid-body (uniform), or differential (non-uniform) with angular velocity increasing toward the poles. Also, the aspects (orientations) of two Be stars, determined from measured line profiles, are consistent with the hypothesis that these stars are rotating near the equatorial break-up velocity.

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