Thorne–Żytkow object

A Thorne–Żytkow object (TŻO or TZO), also known as a hybrid star, is a conjectured type of star wherein a red giant or red 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, located in the Small Magellanic Cloud (SMC), was a strong candidate. Another possible candidate is the star HV 11417, also located in the SMC.
== Formation ==
A Thorne–Żytkow object is formed when a neutron star collides with another 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 when one of the two stars goes 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.
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.
== Observation history ==
In 2014, a team led by Emily Levesque argued that the star HV 2112 had unusually high abundances of elements such as molybdenum, rubidium, lithium, and calcium, and a high luminosity. Since both are expected characteristics of Thorne–Żytkow objects, this led the team to suggest that HV 2112 might be the first discovery of a TZO. However, this claim was challenged in a 2018 paper by Emma Beasor and collaborators, who argued that there is no evidence for HV 2112 having any unusual abundance patterns beyond a possible enrichment of lithium and that its luminosity is too low. They put forth another candidate, HV 11417, based on an apparent over-abundance of rubidium and a similar luminosity as HV 2112. However, HV 11417 has since been identified as a likely foreground halo star.
== TZO candidates ==
* HV 2112 (Small Magellanic Cloud) - Classified as a supergiant TZO candidate or an AGB star
* HV 11417 (Small Magellanic Cloud) - Classified as an AGB star or a foreground halo star
* V595 Cassiopeiae
* IO Persei
* KN Cassiopeiae
* U Aquarii - This star was catalogued as a R Coronae Borealis variable.
* VZ Sagitarii - This star was catalogued as a R Coronae Borealis variable.
== See also ==
* Neutron star
* Red giant
* Quasar
* Quasi star
Category:Stars
Category:Star types