AThorne–Żytkow object (TŻO orTZO), a type of hybrid star, is a conjectured type ofstar wherein ared giant orred supergiant contains aneutron star at its core, formed from thecollision of the giant with the neutron star. Such objects were hypothesized byKip Thorne andAnna Żytkow in 1977.[1] In 2014, it was discovered that the starHV 2112, located in theSmall Magellanic Cloud (SMC), was a strong candidate,[2][3] though this view has since been refuted.[4] Another possible candidate is the starHV 11417, also located in the SMC.[4]
A Thorne–Żytkow object would be formed when aneutron star collides with anotherstar, often a red giant or supergiant. The colliding objects can simply be wandering stars, though this is only likely to occur in extremely crowdedglobular clusters. Alternatively, the neutron star could form in abinary system when one of the two stars goessupernova. Becauseno supernova is perfectly symmetric, and because thebinding 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 amain-sequence star, it may be engulfed when its companion evolves into a red giant.[5]
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'sorbit 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 theTolman–Oppenheimer–Volkoff limit, then the two will collapse into ablack hole. Otherwise, the two will coalesce into a single neutron star.[6]
If a neutron star and awhite dwarf merge, this could form a Thorne–Żytkow object with the properties of anR Coronae Borealis variable.[7]
The surface of the neutron star is very hot, with temperatures exceeding 109K, hotter than the cores of all but the most massive stars. This heat is dominated either bynuclear fusion in the accreting gas or by compression of the gas by the neutron star's gravity.[8][9] 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 ofisotopes than it does in ordinarystellar nucleosynthesis, and some astronomers have proposed that therapid proton nucleosynthesis that occurs in X-ray bursts also takes place inside Thorne–Żytkow objects.[10]
Observationally, a Thorne–Żytkow object may resemble ared supergiant,[2] or, if it is hot enough to blow off the hydrogen-rich surface layers, a nitrogen-richWolf–Rayet star (type WN8).[11]
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 theMilky Way.[12]
The only way to unambiguously determine whether or not a star is a TŻO is a multi-messenger detection of both thegravitational waves of the inner neutron star and an optical spectrum of the metals atypical of a normalred supergiant. It is possible to detect pre-existing TŻOs with currentLIGO detectors; the neutron star core would emit a continuous wave.[13]
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 massiveaccretion disk.[14] These neutron stars may form the population of isolatedpulsars with accretion disks.[14] 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 ablack hole.[15]
In 2014, a team led byEmily Levesque argued that the starHV 2112 had unusually high abundances of elements such asmolybdenum,rubidium,lithium, andcalcium, 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.[2] However, this claim was challenged in a 2018 paper byEmma Beasor and collaborators,[4] 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.[4]
| Candidate | Right ascension | Declination | Location | Discovery | Notes | Refs |
|---|---|---|---|---|---|---|
| HV 2112 | 01h 10m 03.87s | −72° 36′ 52.6″ | Small Magellanic Cloud | 2014 | Classified as a supergiant TZO candidate[2][16][17][18] or anAGB star[4] | [2][16][17][18][4] |
| HV 11417 | 01h 00m 48.2s | −72° 51′ 02.1″ | Small Magellanic Cloud | 2018 | Classified as an AGB star[4] | [4][19] |
| VX Sagittarii | 18h 08m 04.04831s | −22° 13′ 26.6327″ | Sagittarius | 2021 | Classified as red supergiant, red hypergiant or a possible super-AGB star[20] | [20][21] |
| V595 Cassiopeiae | 01h 43m 02.72s | +56° 30′ 46.02″ | Cassiopeia | 2002 | [22] | |
| IO Persei | 03h 06m 47.27s | +55° 43′ 59.35″ | Perseus | 2002 | [22] | |
| KN Cassiopeiae | 00h 09m 36.37s | +62° 40′ 04.12″ | Cassiopeia | 2002 | [22] | |
| U Aquarii | 22h 03m 19.69s | −16° 37′ 35.2″ | Aquarius | 1999 | Catalogued as aR Coronae Borealis variable | [7] |
| VZ Sagittarii | 18h 15m 08.58s | −29° 42′ 29.6″ | Sagittarius | 1999 | Catalogued as aR Coronae Borealis variable | [7] |
| Candidate former TŻO | Right ascension | Declination | Location | Discovery | Notes | Refs |
|---|---|---|---|---|---|---|
| GRO J1655-40 | 16h 54m 00.14s | −39° 50′ 44.9″ | Scorpius | 1995 | The progenitor for both the companion star and the black hole in this system is hypothesized to have been a TŻO. | [15] |
| BD+61 2536 (TIC 470710327) | 23h 49m 18.99s | +61° 57′ 46.0″ | Cassiopeia | 2022 | Massive hierarchical triple star system that could evolve either into aneutron-star merger or a TŻO. | [23] |
| Formation | |||||||
|---|---|---|---|---|---|---|---|
| Evolution | |||||||
| Classification |
| ||||||
| Nucleosynthesis | |||||||
| Structure | |||||||
| Properties | |||||||
| Star systems | |||||||
| Earth-centric observations | |||||||
| Lists | |||||||
| Related | |||||||
