A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.
It is well known that massive stars can collapse to form neutron stars, under extreme temperatures and pressures. In simple terms, neutrons usually have space separating them, due to degeneracy pressure keeping them apart. Under extreme conditions such as a neutron star, the pressure separating nucleons is overwhelmed by gravity, and the separation between them breaks down, causing them to be packed extremely densely and form an immensely hot and dense state known as neutron matter. Because these neutrons are made of quarks, it is hypothesized that under even more extreme conditions, the degeneracy pressure keeping the quarks apart within the neutrons might break down in much the same way, creating an ultra-dense phase of degenerate matter based on densely packed quarks. This is seen as plausible, but is very hard to prove, as scientists cannot easily create the conditions needed to investigate the properties of quark matter, so it is not yet certain whether or not it actually happens in the universe.
If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons (which are formed from quarks bound together) break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.
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, they would 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 not have typical size, electromagnetic, or temperature measurements, compared to other neutron stars.
The hypothesis 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 equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K can not be recreated artificially, so there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.
Creation
It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.[3][4]Stability and strange quark matter
Ordinary quark matter consisting of up and down quarks (also referred to as u and d quarks) has a very high Fermi energy compared to ordinary atomic matter and is only stable 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 dissolve spontaneously.[5][6]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 u and d quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle.[5] This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter.[7]
Strange stars
Quark stars made of strange quark matter are known as strange stars, and they form a subgroup under the quark star category.[7]Strange stars might exist without regard of the Bodmer–Witten assumption of stability at near-zero temperatures and pressures, as strange quark matter might form and remain stable at the core of neutron stars, in the same way as ordinary quark matter could.[3] Such strange stars will naturally have a crust layer of neutron star material. The depth of the crust layer will depend on the physical conditions and circumstances of the entire star and on the properties of strange quark matter in general.[8] Stars partially made up of quark matter (including strange quark matter) are also referred to as hybrid stars.[9][10][11][12]
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.[5] 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.[5]
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 Fermi liquid and enter a so-called color-flavor-locked (CFL) phase of color superconductivity, where "color" refers to the six "charges" exhibited in the strong interaction, instead of the positive and the negative charges in electromagnetism. At slightly lower densities, corresponding to higher layers closer to the surface of the compact star, the quark matter will behave as a non-CFL quark liquid, a phase that is even more mysterious than CFL and might include color conductivity and/or several additional yet undiscovered phases. None of these extreme conditions can currently be recreated in laboratories so nothing can be inferred about these phases from direct experiments.[13]
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 the strong force that binds ordinary hadrons.
Strange stars
Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like—nearly indistinguishable from—ordinary neutron stars. This suggests that many, or even all, known neutron stars might in fact be strange stars. However, the investigating team of Prashanth Jaikumar, Sanjay Reddy, and Andrew W. Steiner made some fundamental assumptions that led to uncertainties in their results large enough that the case is not finally settled. More research, both observational and theoretical, remains to be done on strange stars in the future.[14]Other theoretical work[15] contends that, "A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star"; and, addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars.
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.Quark stars and strange stars are entirely hypothetical as of 2018, but there are several candidates.
Observations released by the Chandra X-ray Observatory on April 10, 2002 detected two possible quark stars, designated RX J1856.5-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than it should be, suggesting that they are composed of material denser than neutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive;[16] and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded.
Another star, XTE J1739-285,[17] 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, Y. L. Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star.[18]
It was reported in 2008 that observations of supernovae SN2006gy, SN2005gj and SN2005ap also suggest the existence of quark stars.[19] It has been suggested that the collapsed core of supernova SN1987A may be a quark star.[20][21]
In 2015, Z.G. Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.[22]
Other theorized quark formations
Apart from ordinary quark matter and strange quark matter, other types of quark-gluon plasma might theoretically occur or be formed inside neutron stars and quark stars. This includes the following, some of which has been observed and studied in laboratories:- Jaffe 1977, suggested a four-quark state with strangeness (qsqs).
- 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, Ө+.
- This can also be described by the diquark model of Jaffe and Wilczek (QCD).
- Ө++ and antiparticle Ө−−.
- Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
- Charmed pentaquark Өc(3100) (uuddc) state was detected by the H1 collaboration.[23]
- Tetra quark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetra quark particle of Z(4430), was discovered and investigated in laboratories on Earth.[24]
