Toward seismology of δ Scuti stars

Many among δ Scuti variables were shown to be multiperiodic and all of them probably are. Being in various stages of early evolutionary phase these stars seem excellent candidates for seismic testing the basic assumptions and physics of the stellar evolution theory. What hinders any major progress in the field is an unresolved problem of connecting sparse spectra of the observed oscillation frequencies to dense spectra calculated for the models. Determination of the rotation rate in the interior is probably the most important goal of asteroseismology. In the case of δ Scuti stars there is a complication following from the high density of the spectrum that the standard formula for the rotational splitting may not be valid. To understand mode selection mechanism we have to go beyond the linear adiabatic theory of stellar oscillations. There are still uncertainties in determining mode stability, but the real difficulty lies in prediction which of many unstable models may reach detectable amplitudes. Trapping in the envelope is a possible mechanism of mode selection, but it is not clear yet whether it finds support in observational data. Observational information on spherical harmonics associated with the observed periodicities is crucial for mode identification. This was obtained with use of both photometric and spectroscopic data for number of δ Scuti stars. There are, however, uncertainties which must be clarified with new data and improved methods of their analysis. Efforts in obtaining periodograms for unevolved objects having simple theoretical frequency spectra are encouraged.