Observational predictions for Thorne–Zytkow objects

Model data, input files, and plotting scripts for the paper : Observational predictions for Thorne–Zytkow objects

Abstract

Thorne–Zytkow objects (TZO) 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 TZOs with the MESA stellar evolution code. With these models, we predict several observational properties of TZOs, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible TZO solutions to cover $3.45\( \lesssim\) log (T/Teff) \( \lesssim\) 3.65 and 4.85 \( \lesssim\)log (L/L_sun) \( \lesssim\)5.5. Due to the much higher densities our TZOs reach compared to previous models, if TZOs 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 TZOs should have fundamental pulsation periods of 1000--2000 days, and period ratios of ~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 TZO: the isotopologues  \(^{44}\rm{Ti O}\) and \(^{44}\rm{Ti O_2}\), 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) TZOs show little heavy metal enrichment, potentially explaining the difficulty in finding TZOs to-date.

MESA version: r22.11.1