Thorne-Żytkow objects have rejuvenated rapidly moving progenitors

When a massive star with a neutron star companion engages in a common envelope phase, there are two possible outcomes: formation of a short-period binary (a potential progenitor of a gravitational-wave source) or a merger. If the binary merges, it can form a Thorne-Żytkow Object (TŻO), an exotic star that has a neutron star core surrounded by a stellar envelope. We use population synthesis at Solar metallicity to study the demographics and rates of TŻOs. We find that most TŻO progenitors have experienced mass transfer earlier in their evolution and have therefore become rejuvenated. Recent work shows that rejuvenated stars have less bound envelopes, meaning that the envelope can be more easily ejected compared to stars that have not experienced a previous mass transfer episode. This indicates that rejuvenation could result in lower TŻO rates and higher double compact object formation and merger rates. We find that the vast majority of TŻOs are rejuvenated. Moreover, the systemic velocity distribution shows that the distribution of TŻOs with main-sequence donors has a rapidly moving extended tail not present in the distribution of TŻOs with more evolved donors. Therefore, we conclude that the most promising candidates for TŻOs are cool, luminous, and rapidly moving giants.