Delta Scuti Stars in Eclipsing Binary Systems
Creators: name: Streamer, Margaret
Publisher: name: The Australian National University
Year: 2021

description: Delta Scuti stars are pulsating A- and F-type stars which, in principle have their internal structure and parameters including age, ascertained by their pulsational modes. Despite the vast amount of photometric data from space-based probes, observations are often still not matched with theoretical models. It remains uncertain if there are significant missing physics in the models, or if the observations are insufficient or inaccurate to explain these stellar objects. The difficulties in modelling delta Sct stars are in sharp contrast to the success of asteroseismology in providing exquisite information on the internal structure of the sun and other solar-like oscillators. Two factors significantly contribute to this failure: 1) Rotation rates of single stars are not easily determined and are more challenging in single delta Sct stars where equatorial velocities greater than 300 km/s are often observed. This causes splitting of the p modes, leading to a complex pattern of observed frequencies, confusing the rotational signals normally found in asteroseismic data. 2) The fundamental properties, such as mass and radius and hence density, of any single star are difficult to ascertain with low uncertainties. However, the density of delta Sct stars is a crucial parameter for p mode identification. Both these short-comings are addressed if the delta Sct star is part of an eclipsing binary system. All the fundamental properties of both components are open to analysis. If the binary has a short orbital period, the system is almost certainly tidally locked. This gives the rotation period for the pulsating component to an accuracy unavailable for single stars. Theoretical stellar and evolutionary models can then be tested against the dynamically-derived properties of the pulsator. This is the crucial step required for mode identification of the observed pulsation frequency. This thesis investigates two eclipsing binary systems, TT Horologii and RZ Microscopii, each with a delta Sct-star component discovered by the author. TESS data augmented ground-based data for RZ Mic but were unavailable for the TT Hor publication. Spectroscopy from the 2.3 m telescope at Siding Springs Observatory gave stellar radial velocities and the temperatures of the pulsators. The combined data enabled accurate modelling of the systems and gave the fundamental stellar parameters and the rotation rates of the pulsators. In both cases, mode identification of the observed frequencies was achieved from theoretical models. These studies confirm that short-period, eclipsing binaries are ideal for investigation of delta Sct stars. To be used as calibration systems, they indicate that the density of the pulsator must be known to within 2% uncertainty and the rotation rate to &lt;0.1%for accurate modelling of the observed frequencies. A higher than solar metallicity was needed for successful evolutionary models suggesting some short comings in this procedure. In the future, the combination of the mode spectrum, Gaia distance and projected rotational velocity, may enable the application of work in this thesis to single stars.