Convection and Pulsation in Cool Cluster Giants

Stellar astrophysics has tremendously benefited from helio- and asteroseismology during the last years. However, a test is, still lacking if current standard stellar structure models are able to reproduce the observed oscillation frequencies, and therefore the inner structure, of a star with a-priori known surface properties. Remarkably, there is only the Sun for which such a fundamental test and comparison yet has been made to a sufficient accuracy. Such tests triggered the development of models that fit the solar interior (i.e., the eigenfrequencies) as well as the surface conditions (i.e., the radius, luminosity, etc.) and led to the well-known improvements in the standard solar model. Even in the age of high-precision space missions, there is not a single star for which the surface properties and - at the same time - the eigenfrequencies are known well enough to perform such a test with comparable accuracy as for the Sun. One might expect that very bright solar-type oscillating stars are perfect candidates for such a test as their fundamental parameters are well constrained from ground observations. But their frequencies, on the other hand, are known less accurately, since the stars are not accessible to the high performance space-photometry of, e.g., the French-led satellite CoRoT or the NASA space telescope Kepler. The stars that are accessible to these missions and for which we have excellent frequency accuracy are much fainter and consequently their fundamental parameters, derived from ground, are often ambiguous. A chance to overcome these restrictions to a large extent is provided by stars in clusters. Even though their absolute fundamental parameters may remain uncertain, because the cluster stars themselves are faint and it is difficult to precisely determine, e.g., their distance, the fact that they are cluster members (i.e., they have the same distance, age, etc.) can be used to determine very accurately relative parameters. The first realistic chance to use this cluster bonus in asteroseismology of solar-type oscillations is provided by Kepler, which has already detected solar-type oscillations in about 100 red giants in two different clusters. In this FWF project I aim to determine all basic fundamental parameters differentially and thus arrive at an accuracy, approaching the solar case by modeling an ensemble of solar-type pulsators. The ongoing observations with Kepler will soon allow constraining individual oscillation frequencies that are precise enough for a detailed asteroseismic analysis. These frequencies will then be compared to theoretical eigenspectra of a new large and dense grid of red-giant models. The models ultimately pass the proposed test if the relative fundamental parameters of a set of models that are only constrained from the observed frequencies do also match the independently determined relative fundamental parameters of the corresponding stars. Furthermore, the proposed project will give access to new territory in the modeling of red giants, such as to study the influence of inadequate modeling of the near surface layers, know as the "near-surface effect", or to explore to what extent the mixing length parameter of the convective regions needs to be tuned during stellar evolution calculations to construct more realistic red-giant models.

Goal of this FWF project was to test if current standard stellar structure models are able to reproduce the observed oscillation frequencies, and therefore the inner structure, of a star with a-priori known surface properties. Remarkably, there was only the Sun for which such a fundamental test and comparison yet has been made to a sufficient accuracy. Such tests triggered the development of models that fit the solar interior (i.e., the eigenfrequencies) as well as the surface conditions (i.e., the radius, luminosity, etc.) and led to the well-known improvements in the standard solar model. Even in the age of high-precision space missions, there is not a single star for which the surface properties and - at the same time - the eigenfrequencies are known well enough to perform such a test with comparable accuracy as for the Sun. One might expect that very bright solar-type oscillating stars are perfect candidates for such a test as their fundamental parameters are well constrained from ground observations. But their frequencies, on the other hand, are known less accurately, since the stars are not accessible to the high performance space-photometry of, e.g., CoRoT or Kepler. The stars that are accessible to these missions and for which we have excellent frequency accuracy are much fainter and consequently their fundamental parameters, derived from ground, are often ambiguous. A chance to overcome these restrictions to a large extent is provided by stars in clusters. Even though their absolute fundamental parameters may remain uncertain, because the cluster stars themselves are faint and it is difficult to precisely determine, e.g., their distance, the fact that they are cluster members (i.e., they have the same distance, age, etc.) can be used to determine very accurately relative parameters. The first realistic chance to use this cluster bonus in asteroseismology is provided by Kepler, which has detected solar-type oscillations in about 100 red giants in two different clusters. Within this FWF project we determined all basic fundamental parameters differentially and thus arrived at an accuracy, approaching the solar case by modelling an ensemble of solar-type pulsators. The 4-years long Kepler observations enabled constraining individual oscillation frequencies that are precise enough for a detailed comparison to theoretical eigenspectra of large and dense grids of red-giant models. Using new methods for this comparison did not only allow us to show that the models ultimately pass the proposed test but also to more deeply study known insufficiencies in the modelling of the near surface layers, known as the near-surface effect.