Observational predictions for Thorne-Żytkow objects

Abstract: Thorne-$.Z$ytkow objects (T$.Z$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$.Z$Os with the MESA stellar evolution code. With these models, we predict several observational properties of T$.Z$Os, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible T$.Z$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$.Z$Os reach compared to previous models, if T$.Z$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$.Z$Os should have fundamental pulsation periods of 1000--2000 days, and period ratios of $\approx$0.2--0.3. Models computed with a large 399 isotope fully-coupled nuclear network show a nucleosynthetic signal that is different to previously predicted. We propose a new nucleosynthetic signal to determine a star's status as a T$.Z$O: the isotopologues $^{44}\rm{Ti} \rm{O}_2$ and $^{44}\rm{Ti} \rm{O}$, 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$.Z$Os show little heavy metal enrichment, potentially explaining the difficulty in finding T$.Z$Os to-date.