Editing Thorne–Żytkow object

{{redirect|TZO|the language with ISO 639 code "tzo"|Tzotzil language}}
{{short description|Hypothetical hybrid star type}}

A '''Thorne–Żytkow object''' ('''TŻO''' or '''TZO'''), a type of hybrid star, is a conjectured type of [[star]] wherein a [[red giant]] or [[Red supergiant star|red supergiant]] contains a [[neutron star]] at its core, formed from the [[Stellar collision|collision]] of the giant with the neutron star. Such objects were hypothesized by [[Kip Thorne]] and [[Anna Żytkow]] in 1977.<ref name=tz77>{{cite journal | title = Stars with degenerate neutron cores. I - Structure of equilibrium models | first1 = Kip S. | last1 = Thorne | author1-link = Kip Thorne | first2 = Anna N. | last2 = Żytkow | journal = [[The Astrophysical Journal]]| volume = 212 | issue = 1 | date = 15 March 1977 | pages = 832–858 | doi = 10.1086/155109 | bibcode = 1977ApJ...212..832T}}</ref> In 2014, it was discovered that the star [[HV 2112]], located in the [[Small Magellanic Cloud]] (SMC), was a strong candidate,<ref name=levesque>{{cite journal |author1-link=Emily Levesque|first1=Emily M. |last1=Levesque |first2=Philip |last2=Massey |first3=Anna N. |last3=Zytkow |first4=Nidia |last4=Morrell |date=2014 |title=Discovery of a Thorne–Żytkow object candidate in the Small Magellanic Cloud |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=443 |pages=L94–L98 |bibcode=2014MNRAS.443L..94L |doi=10.1093/mnrasl/slu080 |doi-access=free |arxiv=1406.0001 |s2cid=119192926}}</ref><ref>{{cite web |date=4 June 2014 |title=Astronomers discover first Thorne-Zytkow object, a bizarre type of hybrid star |website=[[PhysOrg]] |url=http://phys.org/news/2014-06-astronomers-thorne-zytkow-bizarre-hybrid-star.html}}</ref> though this view has since been refuted.<ref name=beasor/> Another possible candidate is the star [[HV 11417]], also located in the SMC.<ref name=beasor>{{cite journal |last1=Beasor |first1=Emma |last2=Davies |first2=Ben |last3=Cabrera-Ziri |first3=Ivan |last4=Hurst |first4=Georgia |title=A critical re-evaluation of the Thorne-Żytkow object candidate HV 2112 |journal=Monthly Notices of the Royal Astronomical Society |volume=479 |issue=3 |pages=3101–3105 |date=2 July 2018 |doi=10.1093/mnras/sty1744 |doi-access=free |arxiv=1806.07399 |bibcode=2018MNRAS.479.3101B |s2cid=67766043 }}</ref>

== Formation ==

A Thorne–Żytkow object would be formed when a [[neutron star]] collides with another [[star]], often a red giant or supergiant. The colliding objects can simply be wandering stars, though this is only likely to occur in extremely crowded [[globular cluster]]s. Alternatively, the neutron star could form in a [[binary star|binary system]] when one of the two stars goes [[supernova]]. Because [[Supernova#Asymmetry|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.<ref name=brandt95>{{cite journal |last1=Brandt |first1=W. Niel |last2=Podsiadlowski |first2=Philipp |title=The effects of high-velocity supernova kicks on the orbital properties and sky distributions of neutron-star binaries |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=May 1995 |volume=274 |issue=2 |pages=461–484 |bibcode=1995MNRAS.274..461B |doi=10.1093/mnras/274.2.461 |doi-access=free |arxiv=astro-ph/9412023 |s2cid=119408422}}</ref>

Once the neutron star enters the red giant, [[drag (physics)|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.<ref>{{cite book |last1=Oohara |first1=Ken-ichi |last2=Nakamura |first2=Takashi |editor1-last=Evans |editor1-first=Charles R. |editor2-last=Finn |editor2-first=Lee S. |editor3-last=Hobill |editor3-first=David W. |chapter=Three dimensional initial data of numerical relativity |title=Frontiers in Numerical Relativity |date=1989 |publisher=Cambridge University Press |isbn=978-0-521-36666-3 |page=84 |chapter-url=https://books.google.com/books?id=XpzuYzu2ulsC&pg=PA84 |language=en}}</ref>

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]].<ref name=1999ApJ...514..932V>{{cite journal |last1=Vanture |first1=Andrew | last2 = Zucker | first2 = Daniel | last3 = Wallerstein | first3 = George |title=U Aquarii a Thorne–Żytkow Object? |journal=[[The Astrophysical Journal]]|date=April 1999 |volume=514 |issue=2 |pages=932–938 |doi=10.1086/306956 |bibcode=1999ApJ...514..932V |doi-access=free }}</ref>

== Properties ==

The surface of the neutron star is very hot, with temperatures exceeding 10<sup>9</sup> [[Kelvin|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.<ref name=eich89>{{cite journal | last1 = Eich | first1 = Chris | last2 = Zimmerman | first2 = Mark | last3 = Thorne | first3 = Kip | author3-link = Kip Thorne | last4 = Żytkow | first4 = Anna N. | author4-link = Anna N. Żytkow | title = Giant and supergiant stars with degenerate neutron cores | journal = [[The Astrophysical Journal]] | date = November 1989 | volume = 346 | issue = 1 | pages = 277–283 | doi = 10.1086/168008 | bibcode = 1989ApJ...346..277E }}</ref><ref name=cannon92>{{cite journal |last1=Cannon |first1=Robert |last2=Eggleton |first2=Peter |last3=Żytkow |first3=Anna N. |author3-link=Anna N. Żytkow |last4=Podsialowsky |first4=Philipp |title=The structure and evolution of Thorne-Zytkow objects |journal=[[The Astrophysical Journal]] |date=February 1992 |volume=386 |issue=1 |pages=206–214 |doi=10.1086/171006 |bibcode=1992ApJ...386..206C |doi-access=free }}</ref> 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 [[isotope]]s than it does in ordinary [[stellar nucleosynthesis]], and some astronomers have proposed that the [[Rp-process|rapid proton nucleosynthesis]] that occurs in X-ray bursts also takes place inside Thorne–Żytkow objects.<ref name=cannon93>{{cite journal |last=Cannon |first=Robert |title=Massive Thorne–Żytkow Objects – Structure and Nucleosynthesis |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=August 1993 |volume=263 |issue=4 |pages=817–838 |bibcode=1993MNRAS.263..817C |doi=10.1093/mnras/263.4.817 |doi-access=free }}</ref>

Observationally, a Thorne–Żytkow object may resemble a [[red supergiant]],<ref name=levesque/> or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-rich [[Wolf–Rayet star]] (type WN8).<ref>{{cite conference |title=Are Peculiar Wolf-Rayet Stars of Type WN8 Thorne-Zytkow Objects? |first1=Cedric |last1=Foellmi |first2=Anthony F. J. |last2=Moffat |bibcode=2002ASPC..263..123F |arxiv=astro-ph/0607217 |date=2002 |isbn=1-58381-103-6 |book-title=Stellar Collisions, Mergers and their Consequences |conference=ASP Conference Proceedings |volume=263 |editor-first=Michael M. |editor-last=Shara }}</ref>

A TŻO has an estimated lifespan of 10<sup>5</sup>–10<sup>6</sup> years. Given this lifespan, it is possible that between 20 and 200 Thorne-Żytkow objects currently exist in the [[Milky Way]].<ref>{{Cite journal |last1=Podsiadlowski |first1=Philipp |last2=Cannon |first2=Robert C. |last3=Rees |first3=Martin J. |date=May 1995 |title=The evolution and final fate of massive Thorne-Żytkow objects |url=http://adsbit.harvard.edu/full/1995MNRAS.274..485P |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=274 |issue=2 |pages=485–490 |bibcode=1995MNRAS.274..485P |doi=10.1093/mnras/274.2.485 |doi-access=free}}</ref>

The only way to unambiguously determine whether or not a star is a TŻO is a multi-messenger detection of both the [[gravitational waves]] of the inner neutron star and an optical spectrum of the metals atypical of a normal [[red supergiant]]. It is possible to detect pre-existing TŻOs with current [[LIGO]] detectors; the neutron star core would emit a continuous wave.<ref>{{Cite journal |last1=DeMarchi |first1=Lindsay |last2=Sanders |first2=Jax R. |last3=Levesque |first3=Emily M. |date=Apr 2021 |title=Prospects for Multimessenger Observations of Thorne-Żytkow Objects |journal=[[The Astrophysical Journal]] |volume=911 |issue=2 |pages=101 |bibcode=2021ApJ...911..101D |doi=10.3847/1538-4357/abebe1 |arxiv=2103.03887 |doi-access=free }}</ref>

== Dissolution ==

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]].<ref name=1995ApJ...455..598M>{{cite journal |title= A Spin-down Variation in the 6 Second X-Ray Pulsar 1E 1048.1-5937 |journal= Astrophysical Journal |volume=455 |page=598 | last = Mereghetti | first = Sandro |publication-date= December 1995 |doi= 10.1086/176607 |bibcode= 1995ApJ...455..598M |date= 1995 }}</ref> These neutron stars may form the population of isolated [[pulsar]]s with accretion disks.<ref name=1995ApJ...455..598M/> The massive accretion disk may also collapse into a new star, becoming a stellar companion to the neutron star. The neutron star may also accrete sufficient material to collapse into a [[black hole]].<ref name=1995MNRAS.277L..35B/>

== 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.<ref name=levesque/> However, this claim was challenged in a 2018 paper by [[Emma Beasor]] and collaborators,<ref name=beasor/> 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.<ref name="beasor" />    

== List of candidate TŻOs ==

{| class="wikitable"
|- 
! Candidate
! Right ascension
! Declination
! Location
! Discovery
! Notes
!| Refs
|-
| {{nowrap|[[HV 2112]]}}
| {{RA|01|10|03.87}}
| {{DEC|-72|36|52.6}}
| [[Small Magellanic Cloud]]
| 2014 
| Classified as a supergiant TZO candidate<ref name=levesque/><ref name=mcmillan>{{cite journal |title= Gaia DR2 Confirms that Candidate Thorne-Żytkow Object HV 2112 is in the Small Magellanic Cloud |journal= Research Notes of the American Astronomical Society | last = McMillan | first = Paul |publication-date= May 2018 |doi=   10.3847/2515-5172/aac0fb |bibcode= 2018RNAAS...2...18M |date= 2018 |volume= 2 |issue= 2 |page= 18 |s2cid= 125376171 |doi-access= free }}</ref><ref name=tout>{{cite journal |title= HV2112, a Thorne-Zytkow object or a super asymptotic giant branch star. |journal= Monthly Notices of the Royal Astronomical Society | last = Tout | first = Christopher |publication-date= November 2014 |doi= 10.1093/mnrasl/slu131 |bibcode= 2014MNRAS.445L..36T |date= 2014 |volume= 445 |pages= L36–L40 |doi-access= free |arxiv= 1406.6064 }}</ref><ref name=worley>{{cite journal |title= The proper motion of HV2112: a TŻO candidate in the SMC |journal= Monthly Notices of the Royal Astronomical Society | last = Worley | first = Clare |publication-date= June 2016 |doi= 10.1093/mnrasl/slw034 |bibcode=  2016MNRAS.459L..31W |date= 2016 |volume= 459 |issue= 1 |pages= L31–L35 |doi-access= free |arxiv= 1602.08479 }}</ref> or an [[Asymptotic giant branch|AGB]] star<ref name=beasor/>
| <ref name=levesque/><ref name=mcmillan/><ref name=tout/><ref name=worley/><ref name=beasor/>
|-
| {{nowrap|[[HV 11417]]}}
| {{RA|01|00|48.2}}
| {{DEC|-72|51|02.1}}
| [[Small Magellanic Cloud]]
| 2018
| Classified as an AGB star<ref name=beasor/> 
|<ref name=beasor/><ref name="ogrady">{{cite journal |last=O'Grady |first=Anna |date=2020 |title=Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-asymptotic Giant Branch Stars |journal=Astrophysical Journal |publication-date=September 2020 |volume=901 |issue=2 |page=135 |arxiv=2008.06563 |bibcode=2020ApJ...901..135O |doi=10.3847/1538-4357/abafad |s2cid=221140015 |doi-access=free}}</ref>
|-
| {{nowrap|[[VX Sagittarii]]}}
| {{RA|18|08|04.04831}}
| {{DEC|-22|13|26.6327}}
| [[Sagittarius (constellation)|Sagittarius]]
| 2021
| Classified as red supergiant, red hypergiant or a possible super-AGB star<ref name=tabernero>{{cite journal |arxiv=2011.09184 |last1=Tabernero |first1=Hugo M. |last2=Dorda Laforet |first2=Ricardo |last3=Negueruela Díez |first3=Ignacio |last4=Marfil |first4=Emilio Gómez |title=The nature of VX Sagitarii |journal=Astronomy & Astrophysics |year=2021 |volume=646 |pages=A98 |doi=10.1051/0004-6361/202039236 |s2cid=227013580 }}</ref>
| <ref name=tabernero /><ref name=farmer>{{cite journal|arxiv=2305.07337 |doi=10.1093/mnras/stad1977 |doi-access=free |title=Observational predictions for Thorne–Żytkow objects |date=2023 |last1=Farmer |first1=R. |last2=Renzo |first2=M. |last3=Götberg |first3=Y. |last4=Bellinger |first4=E. |last5=Justham |first5=S. |last6=De Mink |first6=S. E. |journal=Monthly Notices of the Royal Astronomical Society |volume=524 |issue=2 |pages=1692–1709 }}</ref>
|-
| {{nowrap|[[V595 Cassiopeiae]]}}
| {{RA|01|43|02.72}}
| {{DEC|+56|30|46.02}}
| [[Cassiopeia (constellation)|Cassiopeia]]
| 2002
|
| <ref name=2002ASPC..263..131K>{{cite conference
 |title= A Spectroscopic Search for Massive Thorne-Żytkow Objects
 |first1=Marc J. |last1=Kuchner |author-link1=Marc Kuchner  |first2=David |last2=Vakil
 |first3=Verne V. |last3=Smith  |first4=David L. |last4=Lambert |author-link4=David L. Lambert
 |first5=Bertrand |last5=Plez  |first6=E. Sterl |last6=Phinney
 |bibcode=2002ASPC..263..131K
 |url=https://aspbooks.org/publications/263/131.pdf
 |conference=Stellar Collisions, Mergers and Their Consequences <!--"A symposium in celebration of the opening of the Rose Center for Earth and Space and the newly rebuilt Hayden Planetarium at the American Museum of Natural History"-->
 |location=American Museum of Natural History, New York
 |date=May 30 – June 2, 2000
 |series=ASP Conference Series |volume=263 <!--Publication year=2002-->
 |editor-first=Michael M. |editor-last=Shara
 |isbn=1-58381-103-6 |pages=131–136
}}</ref>
|- 
| {{nowrap|[[IO Persei]]}}
| {{RA|03|06|47.27}}
| {{DEC|+55|43|59.35}}
| [[Perseus (constellation)|Perseus]]
| 2002
|
| {{r|2002ASPC..263..131K}}
|- 
| {{nowrap|[[KN Cassiopeiae]]}}
| {{RA|00|09|36.37}}
| {{DEC|+62|40|04.12}}
| [[Cassiopeia (constellation)|Cassiopeia]]
| 2002
|
| {{r|2002ASPC..263..131K}}
|-
| {{nowrap|[[U Aquarii]]}}
| {{RA|22|03|19.69}}
| {{DEC|-16|37|35.2}}
| [[Aquarius (constellation)|Aquarius]]
| 1999
| Catalogued as a [[R Coronae Borealis variable]]
| <ref name=1999ApJ...514..932V/>
|-
| {{nowrap|[[VZ Sagittarii]]}}
| {{RA|18|15|08.58}}
| {{DEC|-29|42|29.6}}
| [[Sagittarius (constellation)|Sagittarius]]
| 1999
| Catalogued as a [[R Coronae Borealis variable]]
| <ref name=1999ApJ...514..932V/>
|}

== List of candidate former and future TŻOs ==

{| class="wikitable"
|- 
! Candidate former TŻO
! Right ascension
! Declination
! Location
! Discovery
! Notes
! Refs

|- 
| {{nowrap|[[GRO J1655-40]]}}
| {{RA|16|54|00.14}}
| {{DEC|-39|50|44.9}}
| [[Scorpius (constellation)|Scorpius]]
| 1995
| The progenitor for both the companion star and the black hole in this system is hypothesized to have been a TŻO.
| <ref name=1995MNRAS.277L..35B>{{cite journal| last1 = Brandt | first1 = W. Niel | last2 = Podsiadlowski | first2 = Philipp | last3 = Sigurðsson | first3 = Steinn |year=1995 |title = On the high space velocity of X-ray Nova SCO 1994: Implications for the formation of its black hole |journal = [[Monthly Notices of the Royal Astronomical Society]] |volume=277 |issue=2 |pages=L35–L40 |bibcode= 1995MNRAS.277L..35B |doi = 10.1093/mnras/277.1.L35|doi-access = free }}</ref>
|-
| [[BD+61 2536]] 
(TIC 470710327)
| {{RA|23|49|18.99}}
| {{DEC|+61|57|46.0}}
| [[Cassiopeia (constellation)|Cassiopeia]]
| 2022
| Massive hierarchical triple star system that could evolve either into a [[Neutron star merger|neutron-star merger]] or a TŻO.
| <ref>{{Cite journal |last1=Eisner |first1=Nora L. |last2=Johnston |first2=Cole |last3=Toonen |first3=Silvia |last4=Frost |first4=Abigail J. |last5=Janssens |first5=Soetkin |last6=Lintott |first6=Chris J. |last7=Aigrain |first7=Suzanne |last8=Sana |first8=Hugues |last9=Abdul-Masih |first9=Michael |last10=Arellano-Córdova |first10=Karla Z. |last11=Beck |first11=Paul G. |last12=Bordier |first12=Emma |last13=Cannon |first13=Emily |last14=Escorza |first14=Ana |last15=Fabry |first15=Matthias |last16=Hermansson |first16=Lars |display-authors=etal |date=2022-04-01 |title=Planet Hunters TESS IV: a massive, compact hierarchical triple star system TIC 470710327 |journal=Monthly Notices of the Royal Astronomical Society |volume=511 |issue=4 |pages=4710–4723 |doi=10.1093/mnras/stab3619 |doi-access=free |arxiv=2202.06964 |bibcode=2022MNRAS.511.4710E |issn=0035-8711}}</ref>
|- 
|}

== See also ==
*[[Quasar]]
* [[Quasi-star]]

== References ==
{{reflist|30em}}

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