A Thorne–Żytkow object or TŻO is a hypothetical type of star wherein a red giant or supergiant contains a neutron star at its core. Such objects were hypothesized by Kip Thorne and Anna Żytkow in 1977.[1] Though some objects have been suggested as candidates, none has been confirmed.[2]
A Thorne–Żytkow object is formed when a neutron star collides with a star, typically a red giant or supergiant. The colliding objects can simply be wandering stars. This is only likely to occur in extremely crowded globular clusters. Alternatively, the neutron star could form in a binary system. Because no supernova is perfectly symmetric, the neutron star will be left with a small velocity relative to its original orbit. This kick may cause its new orbit to intersect with its companion, or, if its companion is a main sequence star, be engulfed by its companion when it evolves into a red giant.[3]
Once the neutron star enters the red giant, drag between the neutron star and the outer, diffuse layers of the red giant causes the binary star system's orbit to decay, and the neutron star and core of the red giant spiral inward toward one another. Depending on their initial separation, this process may take hundreds of years. When the two finally collide, the neutron star and red giant core will merge. If their combined mass exceeds the Oppenheimer-Volkoff limit then the two will collapse into a black hole, resulting in a supernova that disperses the outer layers of the star. Otherwise, the two will coalesce into a single neutron star.
The surface of the neutron star is very hot, with temperatures exceeding 109 K: hotter than the cores of all but the most massive stars. This heat is dominated either by nuclear fusion in the accreting gas or by compression of the gas by the neutron star's gravity.[4][5] Because of the high temperature, unusual nuclear processes may take place as the envelope of the red giant falls onto the neutron star's surface. Hydrogen may fuse to produce a different mixture of isotopes than it does in ordinary stellar nucleosynthesis, and some astronomers have proposed that the rapid proton nucleosynthesis that occurs in supernovae also takes place inside Thorne–Żytkow objects.[6]
Observationally, a Thorne–Żytkow object may resemble a red supergiant or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich Wolf-Rayet star (type WN8).
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