Excitation and visibility of high-degree modes in stars

Observational evidence for excitation of non-radial modes in stars is confronted with the results of linear stability surveys for stellar models. We consider various types of pulsators on the upper main sequence as well as stars in the Cepheid strip. Our stability survey covers the whole range of spherical harmonic degrees, l, where instability is found. There is fair agreement between the theoretical instability strip and the location of zeta Oph stars, but the observed and calculated periods do not agree in some stars. We suggest that either pulsation is not responsible for the zeta Oph phenomenon or else there are serious errors in mode identification in these cases. We do not find instability at long periods for early B-type stars, supporting the idea that pulsation is not responsible for the periodic variations in Be stars. The agreement between the observed and calculated periods of high-degree modes in delta Sct stars is not very satisfactory. This is attributed to problems in mode identification. We discuss unstable modes of high degree in Cepheid models as a possible mechanism for the low-amplitude radial velocities seen in some stars within the instability strip. We find, however, that the observed periods are at least a factor of 2 longer than the calculated periods. Finally, we discuss the possibility of observing modes of high degree photometrically. We suggest that a large number of high-degree modes may become detectable by future space-borne photometric missions. The confusion arising from these modes may greatly reduce the value of such observations for asteroseismology. However, they will be very important in studying the mechanism of mode selection.