Thorne-Żytkow object

A Thorne-Żytkow object (also written Thorne-Zytkow object when the Ż character is unavailable) or TŻO is a hypothetical type of star, wherein a red giant, red supergiant, or an O-star contains a neutron star inside it.

There are three possible paths for the formation of a Thorne-Żytkow object.

* A Thorne-Żytkow object may result from a binary star system of appropriate mass and proximity. One of the stars ends its life in a supernova explosion and becomes a neutron star. As the other star reaches the end of its main sequence lifetime and swells into a giant, it engulfs the neutron star.

* A star may collide with a wandering neutron star. This is only likely to occur in the extreme crowding of a globular cluster

* In a binary system consisting of two close, massive stars, one star may undergo a supernova and collapse into a neutron star. No supernova is perfectly symmetrical, and the neutron star will be left with a small velocity relative to its previous orbit. This "kick" may cause its new orbit to intersect its companion star.

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 from 100 - 1000 years. When the two finally collide, one of two things may happen.

If the combined mass of the neutron star and the red giant's core exceeds the Oppenheimer-Volkoff limit (the maximum possible mass of a neutron star, somewhere between two and three times the mass of the sun), then the two collapse together into a black hole, resulting in a supernova that disperses the outer layers of the star. Otherwise, the two merely coalesce into a single neutron star.

The surface of the neutron star is very hot, 10⁹ kelvins, hotter than the cores of all but the most massive stars. This heat is produced both by fusion reactions in the accreting gas, and by compression of the gas by the neutron star's gravity. 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 supernovae also takes place inside Thorne-Żytkow objects.

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

Kip Thorne and Anna Żytkow proposed the existence of such stars in 1977, although no Thorne-Żytkow objects have been identified by astronomers.

ee also

* Red Giant star
* Red Supergiant star
* Wolf-Rayet star

References

* [http://cow.physics.wisc.edu/~ogelman/guide/tz/ Thorne-Zytkow Objects]
* [http://research.amnh.org/scicomp/pdfs/astro/Hurley&Shara2002.pdf The promiscuous nature of stars in clusters] , Jarrod R. Hurley and Michael M. Shara, 2001 (mentions of a TŻO formation from collision)
* [http://www.astro.phys.ethz.ch/staff/schmid/private/PSbinary/vanbeveren98.pdf Massive stars] , D. Vanbeveren, C. De Loore, and W. Van Rensbergen, 1998 (explains of a Wolf-Rayet star from TŻO and supernova kick formation)
* [http://arxiv.org/abs/astro-ph/9505025 Gravitational Radiation during Thorne-Żytkow object formation] , S.N. Nazin, K.A. Postnov, 1995
* [http://adsabs.harvard.edu/abs/1999ApJ...514..932V Is U Aquarii a Thorne-Żytkow Object?] , Vanture, Andrew D.; Zucker, Daniel; Wallerstein, George, 1999