A window into $\delta$ Sct stellar interiors: Understanding the eclipsing binary system TT Hor

Abstract

The semi-detached eclipsing binary system, TT Hor, has a $\delta$ 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 center of mass of TT Hor AB over 2 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 $\approx$1.6 $M_{\odot}$ and that of the accretor as $\approx$1.3 $M_{\odot}$. The initial binary orbit has a similar initial separation to the currently observed separation of 11.4 $R_{\odot}$. 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 $\pm$ 0.2 $M_{\odot}$ $\delta$~Sct pulsator which has accreted $\approx$0.9 $M_{\odot}$ of slightly He-enriched material (mean Delta Y <0.01) from the donor over the last 90 Myr. The best fit from all measured parameters and evolutionary state is for a system metallicity of [M/H] is 0.15. A pulsation model of the primary gives a self-consistent set of modes. Our observed oscillation frequencies match to within 0.3% 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 SNR and continuous observations with TESS, more reliable mode identification due to frequency and amplitude changes during the eclipse is likely.