The data to be obtained with STARS will provide a set of oscillation frequencies for a wide range of stars. The values of the frequencies, the amplitudes and line shapes, and the time variation, contain information about the internal structure of the star and about the excitation and damping mechanisms. The data available will differ in quality from one star to another, depending on the characteristics of the star and on the duration of observation; so too will the information that can be extracted about the internal structure of the star. In this section we summarize the information content for different qualities of observational data.
In the best cases we shall be able to determine accurate frequencies and the rotational splitting of modes with degree for a wide range of values of the order n. Such high-quality data will allow us to impose detailed constraints on the structure of the stellar interior, the location (and in principle the structure) of the boundary of convective regions, the properties of the HeII ionization zone, and hence the entropy in the adiabatic region, the variation of rotation with radius and an indication of any large-scale latitudinal variation in the angular velocity.
In the poorest cases we may be able to distinguish only a few peaks corresponding to modes with and a blend of the modes with and 2. This will permit us to determine at least an average value of the so-called large separation ; even this provides one additional observational constraint on stellar models which can be used for calibrating stellar structure theory. In slightly better cases the small separation can also be determined using a superposed spectrum analysis. In the case of sparse spectra, of the kind currently encountered in Scuti and Cephei stars, one cannot identify the modes from the power spectrum alone. However, if modes of low order are present, simultaneous measurements in the optical and UV permit one to disentangle the contributions to the intensity variations from changes in effective temperature and projected surface area, and thereby learn enough about the geometry of the oscillations to determine (Balona and Stobie, 1979).
If the chemical composition of an individual star were known, and convection were understood, we could determine the mass and age of that star from a knowledge of the values of the parameters and (Fig. 2.1). However, that is not the case. Nevertheless, as we demonstrate below, knowledge of those parameters for a number of stars in a cluster does permit an accurate calibration of the mass and age, and in addition the chemical composition, the distance of the cluster, and of the variation of the mixing-length parameter with stellar properties. Moreover, it tests the models of stellar evolution. The situation is improved further if we can determine two or more values of and by averaging over different frequency intervals.