Observational predictions for Thorne-Żytkow objects

Abstract

Thorne-$\dot{Z}$ytkow objects (T$\dot{Z}$O) are potential end products of the merger of a neutron star with a non-degenerate star. In this work, we have computed the first grid of evolutionary models of T$\dot{Z}$Os with the MESA stellar evolution code. With these models, we predict several observational properties of T$\dot{Z}$Os, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible T$\dot{Z}$O solutions to cover $3.45\lesssim \log \left(T/K\right)\lesssim 3.65$ and $4.85\lesssim \log \left(L/L_{\odot }\right)\lesssim 5.5$. Due to the much higher densities our T$\dot{Z}$Os reach compared to previous models, if T$\dot{Z}$Os form we expect them to be stable over a larger mass range than previously predicted, without exhibiting a gap in their mass distribution. Using the GYRE stellar pulsation code we show that T$\dot{Z}$Os should have fundamental pulsation periods of 1000--2000 days, and period ratios of $\approx$0.2--0.3. Models computed with a large 399 isotope fully-coupled nuclear network show a nucleosynthetic signal that is different to previously predicted. We propose a new nucleosynthetic signal to determine a star's status as a T$\dot{Z}$O: the isotopologues ${}^{44}\mathrm{Ti}{\mathrm{O}}_2$ and ${}^{44}\mathrm{Ti}\mathrm{O}$, which will have a shift in their spectral features as compared to stable titanium-containing molecules. We find that in the local Universe (~SMC metallicities and above) T$\dot{Z}$Os show little heavy metal enrichment, potentially explaining the difficulty in finding T$\dot{Z}$Os to-date.

arXiv comment17 pages, 16 figures, 3 Tables, Accepted MNRAS, Zenodo data available https://doi.org/10.5281/zenodo.4534425

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