Thorne-Zytkow Objects
It is important to note that a TZO has never been observed outside the realm of computer simulations
A Thorne-Zytkow Object is the end result of a merger between a Neutron Star and
an O-Star (Red Giant). The two stars begin as a binary star system, and then the Neutron star (>1.4
solar masses) falls into the Red Giant (10-15 solar masses). This may seem strange that a star could fall into
another star, but when we exam the densities, it doesn't seem so strange at all. The Neutron star is extremely dense
because it is made exclusively of Neutrons. It has a similar demsity of an atomic nucleus. A single tablespoon
of it is comparable to a fraight train filled with bricks. The Red Giant on the otherhand is very large, and so
its mass is distributed over a greater volume, making for a much less dense object. A Red Giant has a density
comparable to that of water. If it is easier, imagine the merger between a Neutron star and a Red Giant as being a rock
sinking in water. In this process however it takes 1,000 years to get to the core and one month to get inside the core.
The motion of the neutron star within the envelope of the red giant is on the order of Mach 3-1.4. TZOs are usually understood
as a neutron star within the core of a red supergiant with the radii of several AU, a low temerpature, and a luminosity
of order 10^5 solar luminosity units.
It is believed that the TZOs are formed at at a rate of 1/500 years to 1/1000 years per 10^11 solar mass
Milky-way-like galaxy. This means that more than 30 TZOs could be formed per year within a 30 Mpc radius.
There are generally two possible outcomes of a neutron star sinking into the depths of a red giant.
It will either become a supernova or a TZO.
The TZOs resulting from High Mass X-Ray Binaries (HMXBs) with short orbital periods will not have a long
life. Red Supergiants with radii and luminosities like those of TZOs have strong mass loss by stellar winds, at
a rate of order 10^-6 to 10^-5 solar masses per year.
So the TZO will lose its 13 solar mass
envelope in about 4,000,000 years or less. WHat will be left is a single old neutron star with a moderate space velocity
at most a few km/s which is much lower than the average pulsars.
The large envelope is separated from its compact core by a thin, 40 meter, energy-generation layer called
the "halo". Matter is squeezed through the halo and into the core at a rate of ~1*10^-8 solar masses per year. The contracting
matter releases its gravitational energy and burns its hydrogen and helium while passing through the halo. When the envelope
mass exceeds ~10 solar masses, the hydrogen-burning shell occures at the halo-envelope interface, and the product of hot, T=10^9 K, non-equilibrium
hydrogen burning are convected directly from the burning shell out to the photosphere, where they should be observable.
The inflow of this mass releases nuclear and gravitational energy, converting it into stellar luminosity, L:
(here M-dot is the rate of mass flow into the core, Q is the efficiency of nuclear burning for converting rest mass into thermal energy,
and GM(c)/R(c)C^2 is the analogous efficiency of gravitational contraction with M(c) and R(c) core mass and radius.)
Unfortunatly all of these extreme halo conditions are thoroughly hidden from the prying
eyes of the astronomers by the huge red-giant envelope.
There are two ways of distinguishing a Throne-Zytkow Object (neutron star core) from a normal red supergiant.
The hydrogen will be burned by a hot, T=10^9K, nonequilibrium CNO-Ne reation network, and presumably will produce very peculiar relative abumdances
of various catalyst isotopes (O-18, O-17, O-16, C-13, C-12, etc.). It may be possible to measure these abundances in the photosphere by observational studies
of molecular band spectra such as the rotational bands of carbon monoxide, vanadium oxide, and titanium oxide. Aside from the chemical composition, the only other distinguishing
external feature of the TZO with a neutron star core is the extreme redness. They will be slightly redder, by 
, but since super red-giants are on the edge
of the Hayashi forbidden region TZOs should be the reddest stars in the universe.
Although a red giant of given luminosity may live 20 times longer if it has a neutron core than if it has a white-dwarf core,
giants with neutron star cores may well be much less abundant in the universe than giants with white-dwarf cores: When massive stars form neutron star cores by gravitational collapse,
their loosely bound, tenuous envelopes probably get ejected. If so the only way the neutron core can become a red giant is by aquiring a new envelope - and the only place
this is likely to happen is in a very close binary systen, by supercritical mass transfer from a companion or by a cannibalistic sinking into the companion's center and eating the companion's core.
The "outer region" contains the atmosphere, the photosphere, and the static part of the envelope. The "middle region" contains the inflowing
part of the envelope, the halo, and the outermost layers of the core where the carbon-burning shell is located. The "inner region" is the entire core, except its outermost layers.
The massive envelope of the star can influence the hydrostatic structure of the inner region in only one way: by its weight, which squeezes the inner region to a pressure and density that are higher than
for a bare (envelope-free) neutron star.
At densities above 3*10^11 g/cm^3 ("neutron-drip point") the heat conductivity is so high that the star is very nearly isothermal. Almost all of the core mass is contained in this
isothermal region of the core. Between this isothermal core and the halo (3*10^11>p>10^6 g/cm^3) is a thin insulating layer of degenerate-electron matter which thermally isolates the core from the rest of the star. The center
of this insulating layer has a density of 3*10^8 g/cm^3. Electron degeneracy turns off at a density of 10^6 g/cm^3.
Nuclear burning of inflowing matter will generate heat in the insulating layer of the giant at a rate of:
Electron conductivity cannot carry away much more than ~100 solar luminosity units of this energy; the rest must be carried off by neutrinos.
The middle region acts as a conduit throught which mass flows from the outer region to the inner region. At any given time the total mass in this conduit is ~10^-8 solar masses.
The halo is the region about one meter above the core. At about 15 meters from the core the density drops to ~10^6*e^-15 ~ 1g/cm^3 (the density of water). This rapid density drop cannot continue for many more meters if the star is to support a massive envelope around itself. Due to a profuse turn-on of electron-positron pairs there is a decrease of the mean molecular weight.
The evolution will terminate by collapse of the core to form a black hole when M(tc) reaches the Oppenheimer-Volkoff limit (maximum mass of a neutron star) M(ov)~1.5 - 2 solar masses.
As one might expect, as the core mass grows and the acceleration of gravity at the core edge increases, the thickness of the halo decreases. The total time required for evolution from the initial mass of the core to the point of core collapse, M(ov) is
.
References:
Biehle, G.T. 1991, Astrophysics. J. 380, 167.
Cannon, R., Eggleton, P.P., Zytkow, A.N. and Podsiaklowski, P. 1992 Astrophysics. J. 386, 206.
Kudzitzki, B.P. and Reimers, D. 1978, Astron. Astrophysics. 70, 227.
Thorne, K.S. and Zytkow, A.N. 1977, Astrophysics. J. 212, 832