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After looking for more information about TZOs (Thorne-Zytkow Object), I came up with a question that I had trouble finding answers on the Internet. Do we know of any future TZOs, supergiants being orbited by/orbiting a neutron star whose orbit is decaying at a rate that will cause the supergiant to merge and form a TZO before it dies in a supernova? If so, what are they? If not, why can't we find them?

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One possible formation pathway for TŻOs involves a massive star undergoing mass transfer to a neutron star companion. A common envelope forms, and subsequent friction leads to a merger. High-mass x-ray binaries (HMXBs) are therefore good candidates for progenitor systems. Hutilukejiang et al. 2018 simulated TŻO formation through a number of channels, and found that 3 of 114 known HMXBs in the Milky Way involve a massive supergiant companion capable of filling its Roche lobe and engulfing the neutron star, potentially leading to a TŻO. The three systems listed are SMC X-1, Cen X-3 and LMC X-4.

Progenitors of TŻOs through other channels may be harder (if not impossible) to find. Random collisions between neutron stars and supergiants in globular clusters are effectively unpredictable on an individual basis. Pulsar kicks from a supernova in a supergiant-supergiant system can also lead to TŻOs but are also difficult - again, if not impossible - to identify, given that kicks are not well understood. I would therefore assume that common envelope evolution is the best bet for identifying progenitors.

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