Thorne Żytkow object

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A Thorne-Żytkow Object ( TŻO ) is a hypothetical star in hydrostatic equilibrium with a neutron star at its core . The TŻO is named after the two astronomers Kip Thorne and Anna Żytkow , who gave the first description of such a star.

description

Neutron stars are mainly formed as a result of a supernova that sheds the outer shell of the precursor star. Therefore Thorne-Żytkow objects do not arise in single stars , but only under special circumstances in binary star systems . There are suspected four development channels:

  • A red giant expands so far that a neutron star accompanying it moves its orbit within the common shell . Due to the friction, the neutron star sinks into the core of the red giant and displaces it. However, through accretion, the neutron star would probably form so strong jets that the red giant would be destroyed even before the merger
  • A collision of a neutron star with a massive main sequence star in a star cluster with a high stellar density
  • An asymmetrical supernova explosion drives the newly born neutron star into its companion in a binary star system
  • By mass exchange, the orbit in a reduced X-ray double star consisting of a blue Supergiants and a neutron star to the compact star in the atmosphere of the super giant enters

Thorne Żytkow objects should either be red giants or supergiants with very special properties. Due to the neutron star in their core, they reach a density several orders of magnitude higher than normal stars, which is why different thermonuclear reactions can take place in their interior . These reactions should generate large amounts of lithium and elements from the r-process and p-process . However, it is not clear whether these elements are also transported to the surface, where they can be detected. Theoretical simulations suggest that a Thorne-Żytkow object could develop into a soft gamma repeater or unusual X-ray pulsar . It is assumed that the eruptions of these eruptive stars are caused by sporadic accretion from a fallback disk.

Alternatively, some Wolf-Rayet stars could have formed from Thorne-Żytkow objects. These special WR stars would arise from red giants, in which the additional nuclear reactions and released potential energy have released so much energy that a strong stellar wind has eroded the outer layers of the atmosphere of the red giant.

The lifespan of Thorne-Żytkow objects is probably quite short because the neutron star accretes the former core of its guest star and collapses into a black hole after crossing the Tolman-Oppenheimer-Volkoff boundary . The number of Thorne-Żytkow objects in the Milky Way should be between 20 and 200, assuming a lifespan of 100,000 to 1,000,000 years.

Candidates

The recognized evidence of a Thorne-Żytkow object has not yet been successful. Red giants with unusual chemical abundances of 29 Si and 30 Si such as OH359.762 + 0.120 and U Aquarii could be TŻO. However, stellar evolution models are also conceivable in which the deviating chemical frequencies can arise without the assumption of a TŻO. The same applies to the assumption that HV 2112 could be a TŻO object due to its unusual chemical composition. However, this can also be explained with a conservative model consisting of a super AGB star and an enrichment by a near core collapse supernova.

An alternative proof of the birth of a Thorne-Żytkow object could be provided by gravitational waves . When a TŻO is born, waves with frequencies between 10 −5 and 0.1 Hertz should be emitted, with an amplitude that would be detectable for modern gravitational wave detectors in a radius of 10 kpc . However, this would only mean one detectable event every 500 years.

Individual evidence

  1. KS Thorne, AN Żytkow: Red giants and supergiants with degenerate neutron cores . In: The Astrophysical Journal Letters . tape 199 , July 1975, p. L19 – L24 , doi : 10.1086 / 181839 , bibcode : 1975ApJ ... 199L..19T .
  2. XW Liu, RX Xu, GJ Qiao, JL Han, ZW Han: The extremely long period X-ray source in RCW 103: a descendant of Thorne-Zytkow Object? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.4867v1 .
  3. Oded Papish, Noam Soker, Inbal Bukay: Ejecting the envelope of red supergiant stars with jets launched by an inspiraling neutron star . In: Astrophysics. Solar and Stellar Astrophysics . 2014, arxiv : 1309.3925v3 .
  4. Jump up ↑ MJ Kuchner, D. Vakil, VV Smith, DL Lambert, B. Plez, ES Phinney: A Spectroscopic Search for Massive Thorne-Zytkow Objects . In: Michael M. Shara (Ed.): Stellar Collisions, Mergers and their Consequences (=  ASP Conference Proceedings . Volume 263 ). 2002, ISBN 1-58381-103-6 , pp. 131 .
  5. Jing Wang, Hsiang-Kuang Chang: Neutron star high mass binaries as the origin of SGR / AXP . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.5391v1 .
  6. C. Foellmi, AFJ Moffat: Are Peculiar Wolf-Rayet Stars of Type WN8 Thorne-Zytkow Objects? In: Michael M. Shara (Ed.): Stellar Collisions, Mergers and their Consequences (=  ASP Conference Proceedings . Volume 263 ). 2002, ISBN 1-58381-103-6 , pp. A123 , arxiv : astro-ph / 0607217 .
  7. P. Podsiadlowski, RC Cannon, MJ Rees: The evolution and final fate of massive Thorne-Zytkow objects . In: Monthly Notices of the Royal Astronomical Society . tape 274 , 1995, pp. 485-490 .
  8. ^ J. van Paradijs, HC Spruit, HJ van Langevelde, LBFM Waters: Strategies for detecting Thorne-Zytkow objects . In: A&A . 303, 1995, p. L25. bibcode : 1995A & A ... 303L..25V .
  9. A. Vanture: Is U AQR a Thorne-Zytkow Object? In: News Letter of the Astronomical Society of New York . tape 4 , 1995, p. 6 .
  10. Christopher A. Tout, Anna N. Zytkow, Ross P. Church, Herbert HB Lau: HV2112, a Thorne-Zytkow Object or a Super Asymptotic Giant Branch Star . In: Astrophysics. Solar and Stellar Astrophysics . 2014, arxiv : 1406.6064v1 .
  11. Efrat Sabach, Noam Soker: A super asymptotic giant branch star enriched with calcium by a supernova as the origin of HV2112, rather than a Thorne-Zytkow Object . In: Astrophysics. Solar and Stellar Astrophysics . 2014, arxiv : 1410.1713v1 .
  12. SN Nazis, KA Postnov: Gravitational Radiation during Thorne-Zytkow object . In: A&A . 303, 1995, p. 789. bibcode : 1995A & A ... 303..789N .