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Quark Stars

Quark Star A quark star (Figure 08-20a) is a hypothetical type of star composed of quarks. This is an ultra-dense phase of matter that is theorized to form inside particularly massive neutron stars. It is theorized that when the neutronium, which makes up a neutron star is put under sufficient pressure due to the star's gravity, the individual neutrons break down and their constituent quarks form strange matter. The star then becomes known as a "strange star" or "quark star". Strange matter is composed of up quarks, down quarks and strange quarks bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strange star is essentially a single gigantic nucleon. A quark star lies between neutron stars and black holes in terms of both mass and density, and if sufficient additional matter is added to a strange star it will collapse into a black hole as well.

Figure 08-20a Quark Star
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Despite supposed sightings of quark stars, astronomers remain sceptical about their existence. However, recent theoretical model of quark star shows that such star may collapse in two phases corresponding to two successive bursts of gamma-rays. The first occurs when ordinary matter turns into quark matter. This is followed by a second burst as the quark matter converts to superconducting quark matter. There is a time lapse between the two phases, and the second step releases more energy than the first. Analysis of more than 2000 GRBs identifies 37 bursts that went silent for more than 40 seconds; then started up again and lasted about twice as long as the first. If the quark star model is finally confirmed by further observations, it will be able to resolve three of the astronomical puzzles:
  1. The origin of gamma-ray bursts.
  2. The energy requirement for the r-process, which make elements heavier than zinc.
  3. The two velocity groups with ~125 km/sec and ~ 700 km/sec respectively for the neutron stars - they would be produced by either the one- or two-burst event when the explosions are off-centre.
On February 23, 1987, a supernova explosion was observed in the Large Magellanic Cloud (Figure 08-20b), now designated as SN1987a. For the type-II supernova process, the neutrino spectrum is always characterized by a short peak of electron neutrinos generated during the neutronization phase carrying 1% of the total energy via the process: p + e- n + , while a second release of neutrinos of all flavours stemming from thermal pair processes (pair annihilation, plasmon decay, photoneutrino and Bremsstrahlung) releasing 99% of the energy. Supernovae are expected to radiate about 3x1053 ergs in the form of neutrinos, half of this within 2 seconds, the other half within less than 1 minute. The SN1987a event was captured by three neutrino detectors around the world - Kamiokande II (in Japan), IMB (Irvine-Michigan-
SN1987a SN1987a Data Brookhaven in the US) and Baksan neutrino observatory (in the Caucasus). Their data are shown in Figure 08-20c. A research report in 2009 indicates that there is a significant time delay between the two bursts. It is suggested the first burst was released when a neutron star formed, while the second was triggered seconds later by its collapse into a quark star. High-resolution X-ray observatories, due to fly in space in the next decade, may be able to verify such claim. Neutron stars and quark stars should look very different at X-ray wavelengths.

Figure 08-20b SN1987a
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Figure 08-20c SN1987a Data
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SN2009ip/2010mc The two supernovea SN2009ip and SN2010mc have been observed to have double peaks light curves separated by 40 days as shown in Figure 08-20d. One of the explanations suggests that the second peak indicates the formation of a quark star which cast off the outer crust in the process. The ejecta slams into the debris from the first explosion causing the second peak. According to a model, the 40 days separation is about right. The two peaks will merge if the second explosion happens too soon, and if the detonation of the quark nova occurs too late the debris would have dispersed with no second peak produced. Such model predicts the presence of elements heavier than atomic mass 130 from the second explosion. It would also prove that quarks can exist freely at hight densities and low temperatures rather than always bound up in hadrons.

Figure 08-20d SN2009ip/
2010mc[view large image]

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