Asteroseismology in the red clump: mass transfer and merger remnants through mixed modes

van Rossem, Walter E. ORCID: 0000-0002-8605-5285 (2023). Asteroseismology in the red clump: mass transfer and merger remnants through mixed modes. University of Birmingham. Ph.D.

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Abstract

Asteroseismology enables the study of the interiors of stars through the detection and interpretation of their global oscillation modes. The high-quality observations by the \Kepler space telescope of thousands of giant stars have opened the door to detailed investigations of their structures, facilitating numerous avenues of research that give key insights into stellar structure and evolution. In this context, a particularly exciting prospect offered by seismic constraints is to exploit the probing power of so-called mixed modes. Mixed modes are oscillation modes with both a pressure-mode and a gravity-mode character. Pressure-modes typically propagate in the envelope of a star, whilst gravity-modes do so in the dense radiative regions near the stellar core. The mixed-mode coupling coefficient describes how easily the two types of oscillations can transfer energy between them. The aim of this thesis is twofold. The first aim is to provide a better understanding of mixed-mode coupling in stars undergoing core-helium burning, as well as contributing to the interpretation of mixed-mode coupling in giant stars. The second aim is to search for candidate stars that have undergone mass transfer or mergers in their past. In this thesis stars are modelled using the stellar evolution code \MESA, which enables the inherent complexity of stars to be simulated. Detailed evolutionary models enable stellar properties to be calculated precisely, and the physical relation between these properties to be investigated.The behaviour of model-predicted mixed-mode coupling coefficients in this thesis reproduces that of observed data, in particular its dependence with mass and metallicity. In both the red clump and red giant branch the mixed-mode coupling coefficient is anti-correlated with metallicity for both observed and modelled stars, with the dependence being of the order of $-0.1~\mathrm{dex}^{-1}$ in the red clump and $-0.01~\mathrm{dex}^{-1}$ in the red giant branch. The modelled coupling coefficient depends on the oscillation frequency in both the red giant branch and the red clump of the order of $10^{-3} \mu\mathrm{Hz}^{-1}$. In the red giant branch, this frequency dependence provides an avenue for detailed mapping of the evanescent zone. In the second part of this thesis parametric models are constructed using the stellar evolution code \MESA, allowing for the approximation of mass transfer/merger models. Based on this approximation it is possible to study the effect of the helium core on various observational parameters such as period spacing, luminosity and coupling. From the 1029 stars in our sample of observations, two candidate stars, KIC4275220 and KIC6047033, are identified as candidates of objects which may have had non-standard evolutionary histories.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):