Stellar seismology

We can only see the outer parts of a star: the inner parts are hidden. Yet the important cooking of the elements takes place in the invisible interior, as does the important energy generation. To understand stellar structure and evolution we need to apply advanced indirect methods to `look` into the interior. One of the most important modern methods is the study of stellar oscillations (Astero- or Stellar Seismology). As with the related field of Solar (Helio-)Seismology, it is necessary to reliably detect a large number of excited oscillation frequencies in the selected stars, determine the type of oscillation for each oscillation, and to tune the theoretical models to fit the frequencies. The detection of the oscillation frequencies is accomplished by linking a large number of telescopes around the world to monitor the small light variations. This network has been named the Delta Scuti Network, after the type of star studied most by the network. A specific statistical package extracts and separates the different oscillations. We hope to obtain 100+ frequencies even in stars in the hydrogen-burning phase (`the stellar adulthood`), for which the oscillations are especially difficult to study due to the small amplitudes. Variations in the spectral line properties are used to identify the nature of each oscillation. Examples are the detetion of oscillation wave running around the star with and against the direction of stellar rotation. This motion shows up clearly in the spectral lines. This is all the more remarkable since the star is only a pinpoint of light and the analysis leads to a three-dimensional picture of the surface. The final step in our investigations is the theoretical modeling of the observed variations. Since the observed oscillations depend on the properties of the whole star, including the interior, we can derive tight constraints on the internal structure and tackle complex problems such as the amount of overshooting in the convective core of the star. The astronomers belonging to the Vienna group provide the required expertise in each of these areas in one research institution.

In this project we have probed the interiors of stars by measuring and modelling the multitude of excited stellar oscillations in specially chosen stars. In the field of stellar seismology, the stellar insides, which are hidden from view, can only be studied through indirect methods such as stellar oscillations. The stellar oscillations depend on the physical and chemical conditions of every layer in the star. Consequently, very accurate stellar measurements lead to realistic models of the stars. The present project utilized approximately 1000 nights of telescope time at international sites with excellent atmospheric conditions. Both photometric and spectroscopic measurements of selected Delta Scuti stars were obtained. The data led to a record number of detected pulsation frequencies from earth-based telescopes. However, to be astrophysically useful for detailed stellar modelling, additional information of the pulsation modes connected with individual frequencies is required: the mode identification. This was achieved by measuring with two filters using an important diagnostic tool: the phase differences and amplitude ratios of the light variations in the two passbands. An additional benefit of the two filters was that the reliability of the detected frequencies could be confirmed this way. The measurements were obtained over four years: the resulting high frequency resolution permitted the detection of separate pulsation modes with very close frequencies. This also made it possible to cleanly separate the combination frequencies from the `independent` pulsation modes, which is also required for accurate modelling. Essentially no time shifts between the variations of the amplitudes of the parent modes and the combination frequencies were detected: this argues against resonant excitation of combination modes and supports the explanation of nonlinear changes in the star. Future models need to include these nonlinearities. We obtained detailed astrophysical properties for the Delta Scuti stars 44 Tau, FG Vir and EE Cam with specifics on their chemical composition, evolutionary status and physical processes inside the stars. These results, based on telescope measurements, also have strong implications for the new and future asteroseismic space satellite measurements: the detected clustering of pulsation frequencies of low-degree modes indicate that long measurement runs over several months are required (a warning) and that the evolutionary status of stars with little information can in some cases be obtained from frequency data alone (a positive development). Finally, we have provided statistical packages for the analysis of earth-based and space satellite data.