Present Observational Status of the Intermediate Mass Stars: δ Sct Stars, γ Dor Stars and roAp Stars

Abstract

The present observational status of the δ Sct stars, γ Dor stars and roAp stars is discussed. The δ Sct stars are the most intensively observed of the three groups, but it has become clear that there are severe problems in extracting asteroseismic information from them. Dozens of frequencies are observed, but hundreds of frequencies are predicted from the models; unique matches of observation and theory still elude us. The δ Sct stars are observationally complex — some recent ‘best case’ campaigns are discussed. It is possible that substantial observational advances for δ Sct stars may need to await upcoming satellite missions. New γ Dor stars are being discovered frequently, and new behaviour is being found for them. They constitute an observationally young field. Their pulsational frequency range is being expanded, their position in the HR diagram is becoming better known (but is yet to be fully constrained), and the possibility exists of hybrid γ Dor — δ Sct stars that have great asteroseismic promise, although it is clear such stars are rare, if they do exist. It has been observationally challenging to extract more than a few frequencies for any γ Dor star so far. Exciting spectroscopic discoveries of new behaviour in roAp stars promise unprecedented information about the structure of the peculiar atmospheres of those stars — pulsation amplitude and phase in 3D, magnetic field structure in 3D, abundance stratification in 3D, realistic T-τ for the most peculiar stars — as well as entirely new information about the interaction of pulsation, rotation and magnetic fields. Recent theoretical work has led to new understanding of the previously inexplicable frequency spacing of HR 1217 with new Whole Earth Telescope observations supporting this theory. An ‘improved oblique pulsator model’ has been developed in which the pulsation axis is not the magnetic axis; this model has passed several observational tests and new ones are being devised to examine it further.