Asteroseismic inferences from red-giant stars

Abstract

The analysis of high-precision long-term photometric time-series of data from the NASA Kepler space mission (2008 − 2012) has revolutionized the field of red-giant seismology. Solar-like oscillations could be detected in several thousands of pulsating red-giant stars that cover a wide range of stellar properties and different evolutionary stages. This thesis presents a dedicated study of oscillating red giants that belong to eclipsing binary systems (EBs) and open clusters. Through the study of stellar oscillation modes it is possible to draw inferences on the interior structure and evolution of stars, which is called asteroseismology. In the case of oscillating binary and cluster stars, the stellar parameters can be determined either by analysing the asteroseismic signal or through orbital analysis (EBs), and through stellar isochrones (clusters). The comparison between stellar parameters derived from independent analyses provides the means to test the reliability and accuracy of asteroseismic methods. The research presented in this thesis provides improvements to already existing asteroseismic concepts to allow high-precision asteroseismology. The first part of this thesis focuses on three oscillating red-giant components in EBs. Consistencies between the asteroseismic and dynamical stellar parameters using Kepler’s laws were found in cases where the asteroseismic methods accounted for the stellar mass, temperature, and metallicity dependence, as well as the so-called surface effect. The second part of this thesis discusses an asteroseismic ensemble study with the main objective of determining asteroseismic age estimates of about 60 oscillating red-giant stars that are members of the open clusters NGC 6791 and NGC 6819. Individual age determinations for the cluster giants led to a fairly large age span, although stars in open clusters are assumed to be coeval. The spread in the asteroseismic ages could be reduced by using the clusters’ metallicity from isochrone fitting as constraints for the asteroseismic modelling of the observed cluster giants. The global metallicity of the stars was shown to have a large impact on the age determinations. Improving the accuracy of this parameter would be helpful in obtaining more accurate asteroseismic ages. Among the stars that were targeted by Kepler for open cluster studies was a rare case of a Fourier power density spectrum showing solar-like oscillations of two red giants. The similar asteroseismic ages and masses of the two stars provided indications that they could be physically bound into a rare case of a multiple star system, while different distance estimates could point to an even rarer case of a chance alignment. This study is presented in the third part of this thesis.