Delta Scuti Stars in Eclipsing Binary Systems

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 <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

A window into δ Sct stellar interiors: understanding the eclipsing binary system TT Hor

The semi-detached eclipsing binary system TT Hor has a δ Sct primary component (accretor) accreting mass from the secondary star (donor). We fit an eclipsing binary model from V, B, and I photometry combined with spectroscopy using PHOEBE. Radial velocity variations of the centre of mass of TT Hor AB over two years suggest the presence of a wide companion, consistent with a Kozai–Lidov resonance formation process for TT Hor AB. Evolutionary models computed with MESA give the initial mass of the donor as ≈1.6 M⊙ and that of the accretor as ≈1.3 M⊙. The initial binary orbit has a similar initial separation to the currently observed separation of 11.4 R⊙. Mass transfer commences at an age of 2.5 Gyr when the donor is a subgiant. We model the accretor as a tidally locked 2.2 ± 0.2 M⊙ δ Sct pulsator which has accreted ≈0.9 M⊙ of slightly He-enriched material (mean ΔY < 0.01) from the donor over the last 90 Myr. The best fit from all measured parameters and evolutionary states is for a system metallicity of [M/H] = 0.15. A pulsation model of the primary gives a self-consistent set of modes. Our observed oscillation frequencies match to within 0.3 per cent and the system parameters within uncertainties. However, we cannot claim that our identified modes are definitive, and suggest follow-up time-series spectroscopy at high resolution in order to verify our identified modes. With the higher signal-to-noise ratio and continuous observations with TESS, more reliable mode identification due to frequency and amplitude changes during the eclipse is likely

The 2011 Eruption of the Recurrent Nova T Pyxidis; the Discovery, the Pre-eruption Rise, the Pre-eruption Orbital Period, and the Reason for the Long Delay

We report the discovery by M. Linnolt on JD 2455665.7931 (UT 2011 April 14.29) of the sixth eruption of the recurrent nova T Pyxidis. This discovery was made just as the initial fast rise was starting, so with fast notification and response by observers worldwide, the entire initial rise was covered (the first for any nova), and with high time resolution in three filters. The speed of the rise peaked at 9 mag/day, while the light curve is well fit over only the first two days by a model with a uniformly expanding sphere. We also report the discovery by R. Stubbings of a pre-eruption rise starting 18 days before the eruption, peaking 1.1 mag brighter than its long-time average, and then fading back towards quiescence 4 days before the eruption. This unique and mysterious behavior is only the fourth known anticipatory rise closely spaced before a nova eruption. We present 19 timings of photometric minima from 1986 to February 2011, where the orbital period is fast increasing with P/dot{P}=313,000 yrs. From 2008-2011, T Pyx had a small change in this rate of increase, so that the orbital period at the time of eruption was 0.07622950+-0.00000008 days. This strong and steady increase of the orbital period can only come from mass transfer, for which we calculate a rate of 1.7-3.5x10^-7 Mo/yr. We report 6116 magnitudes between 1890 and 2011, for an average B=15.59+-0.01 from 1967-2011, which allows for an eruption in 2011 if the blue flux is nearly proportional to the accretion rate. The ultraviolet-optical-infrared spectral energy distribution is well fit by a power law with flux proportional to nu^1.0, although the narrow ultraviolet region has a tilt with a fit of \nu^{1/3}. We prove that most of the T Pyx light is not coming from a disk, or any superposition of blackbodies, but rather is coming from some nonthermal source.Comment: ApJ submitted, 62 pages, 8 figures; much added data, updated analysi

Tidally trapped pulsations in a close binary star system discovered by TESS

It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in Transiting Exoplanet Survey Satellite (TESS) space photometry data. The system contains a δ Scuti pulsator in a 1.6 d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Funding for the TESS Asteroseismic Science Operations Centre is provided by the Danish National Research Foundation (grant agreement DNRF106), ESA PRODEX (PEA 4000119301) and Stellar Astrophysics Centre (SAC) at Aarhus University. Some of the observations reported in this paper were obtained with the Southern African Large Telescope (SALT). Polish participation in SALT is funded by grant MNiSW DIR/WK/2016/07. D.W.K. acknowledges financial support from the STFC via grant ST/M000877/1. M.S. is supported by an Australian Government Research Training Program (RTP) Scholarship. G.H., S.C., F.K.A. and P.S. acknowledge financial support by the Polish NCN grant 2015/18/A/ST9/00578. D.J. acknowledges support from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under grant AYA2017-83383-P. We thank the TESS team and staff and TASC/TASOC for their support of the present work and Allan R. Schmitt for making his light-curve examining software LcTools freely available. S.C. is grateful to C. Engelbrecht for introducing him to the use of the observing equipment. G.H. thanks E. Paunzen for helpful discussions on the spectra of λ Boötis stars. A.V. is a NASA Sagan Fellow