The universe is full of unexplainable things and new theories and discoveries emerge every year that have astrophysicists puzzled. Stellar structure has always been a dynamic topic in the field, but in 2006, star XTE J1739-285, was proposed as a candidate for a quark star, a star composed of quark matter. Since then, many other stars have been proposed as candidates, but astrophysicists are still uncertain about the possibility of such bodies to exist. So, do quark stars really exist?
As the name suggests, quark stars are composed of quark matter, or in other words, free quarks. Quarks are one of the fundamental particles in physics. Quarks combine to form hadrons, such as protons or neutrons, and are key to understand atomic and nuclear processes. There are six types of quarks, which are known as flavours: up, down, charm, strange, top and bottom. Protons and neutrons are one of the most stable particles in the universe, and it would take a big amount of energy to separate them into individual quarks. If individual quarks were obtained, it is thought that they would instantly recombine to form another hadron. However, this paradigm would not hold in the case of quark stars, in which quarks are believed to be free.
To understand what quark stars are, we first need to know how they are formed. Stars are formed from a collapsing cloud of gas and dust. Matter starts accumulating at the centre and such high temperatures and pressures are reached that stars begin to fuse hydrogen. When this process is achieved, the star enters the main sequence. The mass of the star will determine the next process once hydrogen in the core has run out, but we are only concerned in those stars that might end up as quark stars. Stars with more mass than half the mass of our Sun, will continue to collapse this time fusing elements up to iron. Once reactions end, gravity makes the star collapse. If the mass of the star is bigger than 8 solar masses, the collapse will end in a supernova. The remnant of the supernova might be either a black hole or a neutron star, or if the hypothesis is correct, a quark star.
Most of the candidates for quark stars are currently classified as neutron stars. Neutron stars are extremely dense objects. A simple teaspoon weighs 10 million tonnes! ( Universe Today, 2016). Gravitational forces are so strong that electrons and protons combine to create neutrons. Neutron stars hold due to neutron degeneracy pressure. Like electrons, two neutrons cannot occupy an identical state even at high pressures. The main reasoning behind quark stars is that they are between neutron stars and black holes, in which they are not massive enough to produce a black hole but have enough mass for neutrons to not be stable. These neutrons would then be broken down into their individual quarks ( Universe Today, 2016). Quark stars are composed with three quark flavours: up, down, and strange. Strange quarks come into play since they are formed when up and down quarks are compressed together.Any net positive quark charge must be balanced with a negative charge, hence electrons. The centres of quark stars are expected to be electrically neutral, hence no presence of electrons would be found. However, if quark stars are more massive, low-density regions will occur around the surface and condensates will be formed ( Weber et al., 2012). A condensate is a state of matter formed when quark gas in low-density regions is cooled to very cold temperatures. These conditions are likely to occur in the surface of the quark stars. Condensates formed in these conditions have been theorised by astrophysicists and all condensates formed contain electrons. The presence of these electrons offers the possibility that quark stars are surrounded by electrons and/or ions, hence having a nuclear crust. The maximum possible density of this crust is estimated to be 4.3 × 1011 g/cm3 ( Weber et al., 2012). The electrons at the surface of the quark star are held electrostatically with the free quarks. This shell of electrons in the surface is only a few fermis thick. Most of the quark stars candidates that have been found have a radius smaller than that of neutron stars. Due to the smaller radius of these objects, these objects obtain very rapid rotation speeds, theorised to be smaller than a millisecond ( Weber et al., 2012).
Since we now know the origin and composition of quark stars, we can now analyse two of the candidates that have been proposed. The first official candidate was neutron star XTE J1739-285. This neutron star is the fastest spinning neutron star known with a frequency of 1122 Hz, which suggests rotation speeds smaller than a millisecond. This star is measured to have a radius between 9 and 12 kilometres and a mass of 1.2 solar masses ( Xiaoping et al., 2007). Models have shown that the only possibility for the star to reach such speeds is if its core is composed of quark matter together with an ionic envelope ( Xiaoping et al., 2007). ASASSN-15lh is the most luminous recently discovered supernova, which could have been triggered by a very fast rotating pulsar. It is theorised that if the pulsar had been a neutron star, the high rotational energy would have quickly dissipated. However, if it had been a star composed of quark matter, this rotational energy would not have dissipated due to the interactions between quarks ( Dai et al., 2018).It is clear to see that quark stars are an ongoing debate in theoretical astrophysics. Several phenomena found in several neutrons star cannot be explained with the current models. However, if the stars were composed of free quarks, models show that these properties could then be explained. Even though these models might work, the existence of free quarks within a body is still a puzzle and the properties of matter at very high densities and very cold temperatures are not yet fully understood. Astrophysics will certainly keep an eye if more candidates for quark stars are found, and hence, decide if they can really exist.
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 Dai et al.(2018). The Most Luminous Supernova ASASSN-15LH: Signature of a Newborn Rapidly-Rotating
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