Observational predictions for Thorne-Zytkow objects

Thorne-.Zytkow objects (T.ZO) 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.ZOs with the MESA stellar evolution code. With these models, we predict several observational properties of T.ZOs, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible T.ZO solutions to cover 3.45 less than or similar to log (T (eff) /K) less than or similar to 3.65 and 4.85 less than or similar to log (L /L (circle dot)) less than or similar to 5.5. Due to the much higher densities our T.ZOs reach compared to previous models, if T.ZOs 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. ZOs should have fundamental pulsation periods of 1000-2000 d, and period ratios of approximate to 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. ZO: the isotopologues (44) TiO2 and (44) TiO, which will have a shift in their spectral features as compared to stable titanium-containing molecules. We find that in the local Universe (similar to SMC metallicities and above) T. ZOs show little heavy metal enrichment, potentially explaining the difficulty in finding T.ZOs to-date.

Automated summary

In this study, the researchers calculated the evolutionary models of T.ZOs and predicted their observational properties such as temperature, luminosity, pulsation period, and nucleosynthetic products. The models suggest that T.ZOs can be stable over a larger mass range without a gap in their mass distribution. A new nucleosynthetic signal, involving isotopologues (44) TiO2 and (44) TiO, is proposed to identify T.ZOs. The study also explains the difficulty in finding T.ZOs based on their low heavy metal enrichment in the local Universe.