WP5 – Asteroseismology

The objectives of the SpaceInn WP5 – Asteroseismology are the following:

The NASA Kepler Mission and the French-led CoRoT satellite have realised huge breakthroughs for the study of stars, in particular those that show solar-like oscillations. Asteroseismic datasets of unprecedented length are now available on thousands of stars, including hundreds of cool main-sequence and sub-giant stars, and on thousands of red giants. The programme will develop and verify robust methodologies to exploit, in a timely manner, the full scientific potential of this ensemble not only for testing stellar evolution theory, but also for providing accurate masses and ages for galactic studies. Another important goal is to fully realize the benefits of exploiting simultaneous ground-based data, from Doppler velocity observations made by large telescopes, the SONG network, and multi-colour campaigns on smaller telescopes.

While the asteroseismic potential of stars considerably more massive than the Sun, whose oscillations are excited by a heat mechanism, is less than for stars with oscillations triggered by stochastic forcing in outer convection zones, the uncertainties on their evolution are far larger. Hence, the relative gain from seismic modelling, even if more limited than for solar-like oscillators, is equally valuable for those objects, particularly if the excited modes can be identified. Mode identification is the largest difficulty for seismic applications to massive stars, particularly when dense frequency spectra of high-order gravity-mode oscillations are detected. On the other hand, the simultaneous detection of self-driven pressure and gravity modes offers the potential to constrain both the inner core regions and the outer layers of the stars, and their successful modelling would have large impact on the stellar evolution theory. Given that the massive stars are mainly responsible for the production of all elements heavier than carbon, and that details of the internal mixing processes determine the amounts of such heavy-element production, massive star asteroseismology is expected to contribute significantly to the understanding of the chemical enrichment of the Galaxy.

Standard stellar models are based on many simplifying hypotheses and often ignore the effects of physical processes such as diffusion, convective overshooting, transport of angular momentum, and rotationally induced mixing in radiative regions. Inaccuracies in the descriptions of these quantities may lead to systematic errors in estimates of the fundamental stellar parameters. Data on stellar oscillations can provide the long-sought additional constraints needed to test the detailed physics of stellar interiors. For example, there are seismic signatures left by the locations of convective boundaries. It is therefore possible to pinpoint the lower boundaries of convective envelopes. These regions are believed to play a key role in stellar dynamos, and so this information is of great importance to stellar dynamo modellers. Furthermore, it is also possible to estimate the sizes of convective cores. Measurement of the sizes of these cores, and the overshoot of the convective motions into the layers above, is important because it can provide an accurate calibration of the ages of the affected stars. The mixing implied by the convective cores, and the possibility of mixing of fresh hydrogen fuel into the nuclear burning cores – courtesy of the regions of overshoot – affects the main-sequence lifetimes.

The substantial uncertainties in our understanding of stellar physics have a direct impact on the calibration of distances on extra-galactic scales, fixing the ages of the oldest stellar populations (which place tight constraints on cosmologies), and tracing the chemical evolution of galaxies. By providing detailed information about stellar interiors asteroseismology will reduce significantly these uncertainties, and therefore strengthen the foundations of these diverse and crucial areas of astrophysics. For example, while the ESA Cornerstone Mission Gaia will give positions and proper motions of stars to unprecedented accuracy, our ability to estimate accurate fundamental stellar properties — such as masses, distances and ages — will remain a limiting factor in our ability to discriminate between different scenarios of formation and evolution of the Galaxy components (halo, thin and thick disk and bulge). Asteroseismology is able to provide such estimates to levels of precision and accuracy, and on stellar samples that are large enough, to overcome these limitations. There are also strong synergies with exoplanet studies. Asteroseismology will allow us to follow stellar cycles and to trace how levels of activity change as stars and their exoplanets age, giving insights on planetary habitability and key information for understanding the variability shown by our own sun.

Exploitation of Space Data for Innovative Helio- and Asteroseismology