Massive Thorne-Żytkow objects (TŻOs) are red supergiants with neutron cores which are expected to form as a result of unstable mass transfer in massive X-ray binaries, or the direct collision of a neutron star with a massive companion after a supernova. We estimate a total TŻO birthrate in the Galaxy of ∼ 2 × 10−4 yr−1. Thus, for a characteristic TŻO lifetime of 105–106 yr, there should be 20-200 TŻOs in the Galaxy at present. These can be distinguished from ordinary red supergiants because of anomalously high surface abundances of lithium and rp-process elements, produced in the TŻO interior. The TŻO phase ends when either the star has exhausted its rp-process seed elements or the envelope mass decreases below a critical mass (∼ 14 M⊙). We confirm earlier conjectures that nuclear burning then becomes inefficient and a neutrino runaway ensues; this can lead to dynamical accretion of matter near the core on to the neutron star and its spin-up to spin periods as low as ∼ 10 ms. The fate of the massive envelope is not entirely clear. If a significant fraction can be accreted on to the core, the formation of a black hole becomes likely. Part of the envelope may collapse into a massive disc, which may ultimately become gravitationally unstable and lead to the formation of planets or even low-mass stars. We discuss the various possible outcomes, and suggest a possible link between massive TŻOs and soft X-ray transients.
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Podsiadlowski et al. (1995) studied this question.