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

Abstract

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.