Thorne–Żytkow object

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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. [1] In 2014, it was discovered that the star HV 2112 was a strong candidate [2] but this has since been called into question. [3]



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 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. [4]

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. Otherwise, the two will coalesce into a single neutron star.[ citation needed ]

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. [5]


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. [6] [7] 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. [8]

Observationally, a Thorne–Żytkow object may resemble a red supergiant, [9] or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich Wolf–Rayet star (type WN8). [10]

A TŻO has an estimated lifespan of 105–106 years. Given this lifespan, it is possible that between 20 and 200 Thorne-Żytkow objects currently exist in the Milky Way. [11]


It has been theorized that mass loss will eventually end the TŻO stage, with the remaining envelope converted to a disk, resulting in the formation of a neutron star with a massive accretion disk. [12] These neutron stars may form the population of isolated pulsars with accretion disks. [12] The massive accretion disk 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. [13]

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, with Emily Levesque being the lead author, noted that HV 2112 displays some chemical characteristics that don't quite match theoretical models, but emphasize that the theoretical predictions for a Thorne–Żytkow object are quite old and theoretical improvements have been made since it was originally conceptualized. [9]

A 2018 paper reappraising the properties of HV 2112, however, has argued that that star is unlikely to be a Thorne-Żytkow object, and it is more likely an intermediate mass AGB star. [3]

List of candidate TŻOs

CandidateRight AscensionDeclinationLocationDiscoveryNotesRefs
HV 11417 2019 [14]
HV 2112 01h 10m 03.87s−72° 36 52.6 Small Magellanic Cloud 2014This star was previously catalogued as an asymptotic-giant-branch star, but observationally is a better fit for red supergiant status. [9]
V595 Cassiopeiae 01h 43m 02.72s+56° 30 46.02 Cassiopeia 2002 [15]
IO Persei 03h 06m 47.27s+55° 43 59.35 Perseus 2002 [16]
KN Cassiopeiae 00h 09m 36.37s+62° 40 04.12 Cassiopeia 2002 [17]
U Aquarii 22h 03m 19.69s−16° 37 35.2 Aquarius 1999This star was catalogued as a R Coronae Borealis variable. [5]
VZ Sagittarii 18h 15m 08.58s−29° 42 29.6 Sagittarius 1999This star was catalogued as a R Coronae Borealis variable. [5]

List of candidate former TŻOs

Candidate former TŻORight AscensionDeclinationLocationDiscoveryNotesRefs
GRO J1655-40 16h 54m 00.14s−39° 50 44.9 Scorpius 1995The progenitor for both the companion star and the black hole in this system is hypothesized to have been a TŻO. [13]

See also

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HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star.

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HV 11417 is a candidate Thorne–Żytkow object. The paper also claims that another candidate for the Thorne–Żytkow object, HV 2112, was not a Thorne–Żytkow object because it seemingly possessed no strange quality that would label it as a Thorne–Żytkow object.


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  2. Levesque, Emily M.; Massey, Philip; Zytkow, Anna N.; Morrell, Nidia (2014). "Discovery of a Thorne–Żytkow object candidate in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society: Letters. 443: L94–L98. arXiv: 1406.0001 . Bibcode:2014MNRAS.443L..94L. doi:10.1093/mnrasl/slu080. S2CID   119192926. Lay summary PhysOrg (4 June 2014).
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  6. Eich, Chris; Zimmerman, Mark; Thorne, Kip; Żytkow, Anna N. (November 1989). "Giant and supergiant stars with degenerate neutron cores". The Astrophysical Journal . 346 (1): 277–283. Bibcode:1989ApJ...346..277E. doi:10.1086/168008.
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  9. 1 2 3 Levesque, Emily; Massey, Philip; Żytkow, Anna; Morrell, Nidia (30 May 2014). "Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society Letters . 1406: L94–L98. arXiv: 1406.0001 . Bibcode:2014MNRAS.443L..94L. doi:10.1093/mnrasl/slu080. S2CID   119192926.
  10. Foellmi, C.; Moffat, A.F.J. (2002). "Are Peculiar Wolf-Rayet Stars of Type WN8 Thorne-Zytkow Objects?". In Shara, Michael M. (ed.). Stellar Collisions, Mergers and their Consequences. ASP Conference Proceedings. 263. arXiv: astro-ph/0607217 . Bibcode:2002ASPC..263..123F. ISBN   1-58381-103-6.
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  12. 1 2 Mereghetti, Sandro (1995). "A Spin-down Variation in the 6 Second X-Ray Pulsar 1E 1048.1-5937". Astrophysical Journal (published December 1995). 455: 598. Bibcode:1995ApJ...455..598M. doi:10.1086/176607.
  13. 1 2 Brandt, W. Niel; Podsiadlowski, Philipp; Sigurðsson, Steinn (1995). "On the high space velocity of X-ray Nova SCO 1994: Implications for the formation of its black hole". Monthly Notices of the Royal Astronomical Society . 277 (2): L35–L40. Bibcode:1995MNRAS.277L..35B. doi: 10.1093/mnras/277.1.L35 .
  14. Beasor, Emma R.; Davies, Ben; Cabrera-Ziri, Ivan; Hurst, Georgia (2018). "A critical re-evaluation of the Thorne–Żytkow object candidate HV 2112" (PDF). Monthly Notices of the Royal Astronomical Society. 479 (3): 3101–3105. arXiv: 1806.07399 . Bibcode:2018MNRAS.479.3101B. doi:10.1093/mnras/sty1744. S2CID   67766043.