Quark star

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A quark star is a hypothetical type of

.

Background

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Some

If these ideas are correct, quark stars might occur, and be observable, somewhere in the universe. Such a scenario is seen as scientifically plausible, but it has been impossible to prove both observationally and experimentally, because the very extreme conditions needed for stabilizing quark matter cannot be created in any laboratory nor has it been observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among the unsolved problems in physics.

If quark stars can form, then the most likely place to find quark star matter would be inside

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or have atypical sizes, electromagnetic fields, or surface temperatures, compared to neutron stars.

History

The analysis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze.^[1][2] Their existence has not been confirmed.

The

It is hypothesized that when the

Ordinary quark matter consisting of up and down quarks has a very high Fermi energy compared to ordinary atomic matter and is stable only under extreme temperatures and/or pressures. This suggests that the only stable quark stars will be neutron stars with a quark matter core, while quark stars consisting entirely of ordinary quark matter will be highly unstable and re-arrange spontaneously.^[6][7]

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of up and down quarks into

Stars made of

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, they could also be created in the early cosmic phase separations following the Big Bang.^[6] If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.^[6]

Characteristics

Quark stars have some special characteristics that separate them from ordinary neutron stars. Under the physical conditions found inside neutron stars, with extremely high densities but temperatures well below 10¹² K, quark matter is predicted to exhibit some peculiar characteristics. It is expected to behave as a

If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than by the strong force that binds ordinary hadrons.^{[citation needed]}

Observed overdense neutron stars

At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low,^{[citation needed]} so in the Milky Way there would only be a small population of quark stars. If it is correct, however, that overdense neutron stars can turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as observers would be looking for the wrong type of star.^{[citation needed]}

A neutron star without deconfinement to quarks and higher densities cannot have a rotational period shorter than a millisecond; even with the unimaginable gravity of such a condensed object the centripetal force of faster rotation would eject matter from the surface, so detection of a pulsar of millisecond or less period would be strong evidence of a quark star.

Observations released by the

Another star, XTE J1739-285,^[11] has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible quark star candidate.

In 2006, You-Ling Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star.^[12]

It was reported in 2008 that observations of supernovae SN 2006gy, SN 2005gj and SN 2005ap also suggest the existence of quark stars.^[13] It has been suggested that the collapsed core of supernova SN 1987A may be a quark star.^[14][15]

In 2015, Zi-Gao Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.^[16]

In 2022 it was suggested that GW190425, which likely formed as a merger between two neutron stars giving off gravitational waves in the process, could be a quark star.^[17]

Other hypothesized quark formations

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Apart from ordinary quark matter and strange quark matter, other types of quark-gluon plasma might hypothetically occur or be formed inside neutron stars and quark stars. This includes the following, some of which has been observed and studied in laboratories:

• Robert L. Jaffe 1977, suggested a four-quark state with strangeness (qsqs).
• Robert L. Jaffe 1977 suggested the H
  dibaryon
  , a six-quark state with equal numbers of up-, down-, and strange quarks (represented as uuddss or udsuds).
• Bound multi-quark systems with heavy quarks (QQqq).
• In 1987, a pentaquark state was first proposed with a charm anti-quark (qqqsc).
• Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqqs).
• Light pentaquarks are grouped within an antidecuplet, the lightest candidate, Θ⁺, which can also be described by the diquark model of Robert L. Jaffe and Wilczek (QCD).
• Θ⁺⁺ and antiparticle Θ⁻⁻.
• Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
• Charmed pentaquark Θ_c(3100) (uuddc) state was detected by the H1 collaboration.^[18]
• Tetraquark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetraquark particle of Z(4430), was discovered and investigated in laboratories on Earth.^[19]

See also

References