## 1. IntroductionThe effects of stellar rotation must be taken into account in
interpreting the spectrum of oscillations of many stars. In the case
of a slow rotator, the effects of rotation are very simply
interpreted: rotation removes the frequency degeneracy in the
azimuthal order An influential paper on the study of oscillations in rotating stars is that by Saio (1981). Saio's numerical results are still in use for interpreting observations. However, Saio's calculations were for simple polytropic models. More realistic models are required for the detailed mode interpretation and seismology of stars that modern observations make a possibility. For this reason, Kjeldsen et al. (1998) calculated stellar models with more realistic physics. This work raised a question about the proper interpretation of Saio's tabulated results, which is addressed in this note. The full calculation of the normal modes of oscillation of the model of a rotating star is quite an undertaking; the usual approach to estimating the frequencies of such a star is therefore to evaluate the frequency changes induced by the effects of rotation, treating them as a perturbation about a nonrotating model. But to make use of such calculations to compare with the observed frequencies of stars, it is necessary to consider carefully what properties of the star are held fixed when making the perturbation. Saio compared the frequencies of rotating and nonrotating polytropes. In that comparison, he kept the central density and central pressure (and hence polytropic constant) fixed. An observationally more relevant comparison, though, would be to keep fixed the observed global properties of the star. This is considered here. © European Southern Observatory (ESO) 1999 Online publication: October 14, 1999 |