Population synthesis of Thorne-Żytkow objects

Context. Common envelope evolution of a massive star and a neutron star companion has two possible outcomes: the formation of a short-period binary (a potential gravitational wave source progenitor) or the merger of the massive star with the neutron star. If the binary merges, a structure may form, comprised of a neutron star core surrounded by a large diffuse envelope, known as a Thorne-Żytkow object (TŻO). The predicted appearance of this hypothetical class of star is very similar to that of a red supergiant, making it difficult to identify them in observations. Aims. Our objective is to understand the properties of systems that are potential TŻO progenitors; specifically, binary systems that enter a common envelope phase with a neutron star companion. We also aim to distinguish those that have been through a previous stable mass transfer phase, which can rejuvenate the accretor. Methods. We used the rapid population synthesis code COMPAS at solar metallicity, with the common envelope efficiency parameter set to unity, to determine the population demographics of TŻOs. We used one-dimensional (1D) evolutionary TŻO models from the literature to determine a fit for the TŻO lifetime to estimate the current number of TŻOs in the Milky Way, as well as to assess core disruption during the merger. Results. We explored the progenitors in the Hertzsprung-Russell diagram, calculated the formation rates, and investigated the kinematics of the progenitor stars. We find that the vast majority (≈92%) of TŻO progenitors in our population have experienced mass transfer and are rejuvenated prior to their formation event. For the Milky Way, we estimate a TŻO formation rate of ≈4 × 104 yr−1, which results in ≈5 ± 1 TŻOs at present.