A Thorne–Żytkow object (TŻO or TZO) is a conjectured type of star wherein a red giant or supergiant contains a neutron star at its core, formed from the collision of the giant with the neutron star. Such objects were hypothesized by Kip Thorne and Anna Żytkow in 1977. In 2014, it was discovered that the star HV 2112 was a strong candidate.
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Formation
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 after one of the two stars went supernova. Because no supernova is perfectly symmetric, and because the binding energy of the binary changes with the mass lost in the supernova, the neutron star will be left with some 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, it may be engulfed when its companion evolves into a red giant.
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 Tolman-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.
If a neutron star and a white dwarf merge, this could form a Thorne–Żytkow object with the properties of an R Coronae Borealis variable.
Properties
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. 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 X-ray bursts also takes place inside Thorne–Żytkow objects.
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).
Dissolution
It has been theorized that the evolution of TŻOs will result in neutron stars with massive accretion discs, as mass loss will end the TŻO stage, and the remaining envelope converts to being a disc. These neutron stars may form the population of isolated pulsars with accretion discs. The massive accretion disc may also result in the collapse of a star, becoming a stellar companion to the neutron star. The neutron star may also accrete sufficient material to collapse into a black hole.
Observation history
As of 2014, the most recent candidate, star HV 2112, has been observed to have some unusual properties that suggest that it may be a Thorne–Żytkow object. The discovering team have noted that HV 2112 displays some chemical characteristics that don't quite match theoretical models, but emphasize that the theoretical predictions for Thorne–Żytkow object are quite old and theoretical improvements have been made since it was originally conceptualized.