A Thorne–Żytkow object (TŻO or TZO) is
a conjectured type of star wherein a red giant
or supergiant contains a neutron star at its
core, formed from the collision of the giant
with the neutron star.
Such objects were hypothesized by Kip Thorne
and Anna Żytkow in 1977.
In 2014, it was discovered that the star HV
2112 was a strong candidate but this has since
been called into question.
== Formation ==
A Thorne–Żytkow object is formed when a
neutron star collides with a star, typically
a red giant or supergiant.
The colliding objects can simply be wandering
stars.
This is only likely to occur in extremely
crowded globular clusters.
Alternatively, the neutron star could form
in a binary system after one of the two stars
went supernova.
Because 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.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 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, resulting in a supernova that disperses
the outer layers of the star.
Otherwise, the two will coalesce into a single
neutron star.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.
== Properties ==
The surface of the neutron star is very hot,
with temperatures exceeding 109 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.
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 X-ray bursts 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).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 the Milky Way.
== 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 disc.
These neutron stars may form the population
of isolated pulsars with accretion discs.
The massive accretion disc may also result
in the collapse of a star, becoming a stellar
companion to the neutron star.
The neutron star may also accrete sufficient
material to collapse into a black hole.
== Observation history ==
As of 2014, the most recent candidate, star
HV 2112, has been observed to have some unusual
properties that suggest that it may be a Thorne–Żytkow
object.
The discovering team, with Emily Levesque
being the lead author, noted that HV 2112
displays some chemical characteristics that
don't quite match theoretical models, but
emphasize that the theoretical predictions
for a Thorne–Żytkow object are quite old
and theoretical improvements have been made
since it was originally conceptualized.A 2018
paper reappraising the properties of HV 2112,
however, has shown that star is unlikely to
be a Thorne-Żytkow object, and it is more
likely an intermediate mass AGB star.
== List of candidate TŻOs ==
== List of candidate former TŻOs ==
== See also ==
Quasar
Quasi-star
