Stellar Insights

Stellar seismology is concerned with the oscillations of stars and what they tell us about the structure and evolution of stars. Astronomical telescopes can only see the surface of the stars. The interior, in which the important `cooking` of the chemical elements is taking place, is hidden from direct observation. Stellar oscillations allow us to probe and construct models of the interior. Fortunately, the majority of stars pulsate with a multitude of different oscillations - each of which contains information on the stellar envelope and interior. Only recently has the astronomical instrumentation become sensitive enough to be able to measure and correctly identify the many individual oscillations in a star. The project will use space satellites and earth-based automatic and classical manual telescopes to measure the oscillations of selected so-called Delta Scuti and Gamma Doradus stars in the hydrogen-burning stage (`the stellar adulthood`) and to use this information to compute theoretical stellar models of stars. In this project we emphasize the detection of a large number of pulsation frequencies with small amplitudes. Special attention will be given to pulsation mode identification: the identification of the individual oscillations with their physical nature (pressure or gravity modes; geometry given by the `quantum numbers of oscillation`). To achieve this, brightness measurements and their variations need to be obtained in more than one wavelength region: we will use the Austrian automatic photoelectric telescope in Arizona, as well as the Austrian/Canadian MOST and BRITE satellites. The stellar oscillations also lead to variations in the spectroscopic line profiles with different pulsation modes having specific signatures. The TOPS team was one of several pioneers in developing special techniques of analysis. Telescope measurements of the line profile variations as well as radial velocities will be obtained and combined with the simultaneous photometry. 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 a number of complex problems such as convection and overshooting, effects of chemical composition, improved opacities, amplitude and period variability, stellar cycles, nonlinearities and hybrid pulsation.

In this project we have probed the inside regions of stars by measuring and modeling 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 the most accurate and extensive measurements of stellar brightness as well as spectroscopic changes available. These were obtained in space as well as on the ground. Our proposed stars were selected to be measured by both the MOST and KEPLER spacecrafts. We also obtained months of telescope time on the McDonald Observatory (Texas, USA). Furthermore, an automatic telescope in Arizona was dedicated to this project. The KEPLER spacecraft measurements increased the experimental precision by a factor of 100 to 1000. Hundreds of stellar oscillations were detected in individual stars. The period spacings allowed these to be classified to belong to specific families. These families, in turn, provided information on the rate of rotation of these stars as well as astrophysical processes active in the stars. An interesting and unexpected discovery was the fact that even simple stars hotter than the Sun show asymmetries across the stellar surface, possibly caused by weak magnetic fields. Furthermore, since the stellar limbs are darker than the center of the stellar disks (as seen from the Earth), it was possible to detect a very large number of rapid waves moving across the stellar disk in a rapidly rotating star. This may have been only the second such detection outside the Sun. The Sun possesses a strong solar cycle with an 11-year (or 22-year magnetic) period. The variation in the strength of these cycles may have a strong effect on the Earth's climate, but the reasons for the change in the stellar cycles are not known. In order to study stellar cycles, we have been measuring the pulsations of a hotter star, 4 CVn, in detail until 2012, when the observing campaign was completed after more than 1000 nights of observation. A strong cycle with systematic period variations (much larger than those of the Sun) is indeed found. The direction of these changes depends strongly on the type of pulsation mode studied. Furthermore, a complete sign reversal was detected around 1990. Our new detailed measurements indicate a contradictory behaviour between radial and nonradial pulsations during the last seven years. The nonradial modes suggest a systematically changing radius of the star, while the constancy of the radial mode rules this out. The contradiction can be resolved if the rotational profile inside the star is changing, i.e. the internal structure of the star is changing within a few years.