The fate of Thorne‐Żytkow objects

We briefly review the processes by which Thorne‐Żytkow objects (TŻOs), i.e., red supergiants with neutron cores, are believed to form. The energy source in massive TŻOs is nuclear burning, provided by a modified rapid p process. After ≲106 yr, this process is expected to break down, leading to a neutrino runaway and the collapse of the TŻO envelope. Part of the envelope will be accreted by the neutron‐star core, which will be spun up and may be transformed into a black hole. The rest of the envelope is likely to form a centrifugally supported disk. This disk will ultimately become gravitationally unstable, possibly forming one or more self‐gravitating objects (planets or low‐mass stars) in the process. The final system may be a spun‐up pulsar surrounded by planets, a low‐mass x‐ray binary, or a low‐mass black‐hole binary like V404 Cygni.