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Quark Stars
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Title: Structure of Quark Stars
Authors: Fridolin Weber, Milva Orsaria, Hilario Rodrigues, Shu-Hua Yang

This paper gives an brief overview of the structure of hypothetical strange quarks stars (quark stars, for short), which are made of absolutely stable 3-flavor strange quark matter. Such objects can be either bare or enveloped in thin nuclear crusts, which consist of heavy ions immersed in an electron gas. In contrast to neutron stars, the structure of quark stars is determined by two (rather than one) parameters, the central star density and the density at the base of the crust. If bare, quark stars possess ultra-high electric fields on the order of 10^{18} to 10^{19} V/cm. These features render the properties of quark stars more multifaceted than those of neutron stars and may allow one to observationally distinguish quark stars from neutron stars.

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Strangelet dwarfs
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Title: Strangelet dwarfs
Authors: Mark G. Alford, Sophia Han, Sanjay Reddy

If the surface tension of quark matter is low enough, quark matter is not self bound. At sufficiently low pressure and temperature, it will take the form of a crystal of positively charged strangelets in a neutralising background of electrons. In this case there will exist, in addition to the usual family of strange stars, a family of low-mass large-radius objects analogous to white dwarfs, which we call "strangelet dwarfs". Using a generic parametrisation of the equation of state of quark matter, we calculate the mass-radius relationship of these objects.

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Quark stars
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Title: Possibility of conversion of neutron star to quark star in presence of high magnetic field
Authors: Ritam Mallick, Monika Sinha

Recent results and data suggests that high magnetic field in neutron stars (NS) strongly affects the characteristic (radius, mass) of the star. They are even separated as a class known as magnetars, for whom the surface magnetic field are greater than 10^{14} G. In this work we discuss the effect of such high magnetic field on the phase transition of NS to quark star (QS). We study the effect of magnetic field on the transition from NS to QS including the magnetic field effect in equation of state (EoS). The inclusion of the magnetic field increases the range of baryon number density, for which the flow velocities of the matter in the respective phase are finite. The magnetic field helps in initiation of the conversion process. The velocity of the conversion front however decreases due to the presence of magnetic field, as the presence of magnetic field reduces the effective pressure (P). The magnetic field of the star gets decreased by the conversion process, and the resultant QS has lower magnetic field than that of the initial NS.

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Cold Quark Matter
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Title: Can cold quark matter be solid?
Authors: Renxin Xu (PKU)

The state of cold quark matter really challenges both astrophysicists and particle physicists, even many-body physicists. It is conventionally suggested that BCS-like colour superconductivity occurs in cold quark matter; however, other scenarios with a ground state rather than of Fermi gas could still be possible. It is addressed that quarks are dressed and clustering in cold quark matter at realistic baryon densities of compact stars, since a weakly coupling treatment of the interaction between constituent quarks would not be reliable. Cold quark matter is conjectured to be in a solid state if thermal kinematic energy is much lower than the interaction energy of quark clusters, and such a state could be relevant to different manifestations of pulsar-like compact stars.

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Title: Cold Quark Matter
Authors: Aleksi Kurkela, Paul Romatschke, Aleksi Vuorinen
(10 Dec 2009)

We perform an O(alpha_sē) perturbative calculation of the equation of state of cold but dense QCD matter with two massless and one massive quark flavour, finding that perturbation theory converges reasonably well for quark chemical potentials above 1 GeV. Using a running coupling constant and strange quark mass, and allowing for further non-perturbative effects, our results point to a narrow range where absolutely stable strange quark matter may exist. Absent stable strange quark matter, our findings suggest that quark matter in compact star cores becomes confined to hadrons only slightly above the density of atomic nuclei. Finally, we show that equations of state including quark matter lead to hybrid star masses up to M~2M_solar, in agreement with current observations. For strange stars, we find maximal masses of M~2.75M_solar and conclude that confirmed observations of compact stars with M>2M_solar would strongly favour the existence of stable strange quark matter.

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Quark stars
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Title: Can stellar mass black holes be quark stars?
Authors: Z. Kovacs, K. S. Cheng, T. Harko

We investigate the possibility that stellar mass black holes, with masses in the range of 3.8 solar masses and 6 solar masses, respectively, could be in fact quark stars in the Colour-Flavour-Locked (CFL) phase. Depending on the value of the gap parameter, rapidly rotating CFL quark stars can achieve much higher masses than standard neutron stars, thus making them possible stellar mass black hole candidates. Moreover, quark stars have a very low luminosity and a completely absorbing surface - the infalling matter on the surface of the quark star is converted into quark matter. A possibility of distinguishing CFL quark stars from stellar mass black holes could be through the study of thin accretion disks around rapidly rotating quark stars and Kerr type black holes, respectively. Furthermore, we show that the radiation properties of accretion disks around black holes and CFL quark stars are also very similar. However, strange stars exhibit a low luminosity, but high temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disk, may be the key signature to differentiate massive strange stars from black hole.

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Title: Strange stars with different quark mass scalings
Authors: Ang Li, Ren-Xin Xu, Ju-Fu Lu

We investigate the stability of strange quark matter and the properties of the corresponding strange stars, within a wide range of quark mass scaling. The calculation shows that the resulting maximum mass always lies between 1.5 solar mass and 1.8 solar mass for all the scalings chosen here. Strange star sequences with a linear scaling would support less gravitational mass, and a change (increase or decrease) of the scaling around the linear scaling would lead to a larger maximum mass. Radii invariably decrease with the mass scaling. While the larger the scaling, the faster the star might spin. In addition, the variation of the scaling would cause an order of magnitude change of the strong electric field on quark surface, which is essential to support possible crusts of strange stars against gravity and may then have some astrophysical implications.

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Title: Quark stars composed of Lennard-Jones matter
Authors: Xiaoyu Lai, Renxin Xu

Quark clustering, rather than colour super-conducting, could occur in cold quark matter because of the strongly coupling between quarks at realistic baryon densities of compact stars. Although one may still not be able to calculate this conjectured matter from first principles, the inter-cluster interaction might be analogised to the interaction between inert molecules. Cold quark matter would then crystallise in a solid state if the inter-cluster potential is deep enough to trap the clusters in the wells. We apply the Lennard-Jones potential to describe the inter-cluster potential, and derive the equations of state, which are stiffer than that derived in conventional realistic models (e.g., MIT bag model). If quark stars are composed of Lennard-Jones matter, they could have high maximum masses (>2 solar masses) as well as very low masses. These features could be tested by observations.

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Recently astronomers from the University of Calgary proposed an explanation for the two brightest supernovas ever observed, designated SN 2005ap and SN 2006gy. The proposed explanation is that they happened when an extremely dense neutron star exploded and became a hypothesized object called a "quark star." SN 2006gy occurred about 240 million light-years away, and thus 240 million years in the past; SN 2005ap was nearly 5 billion light-years away, thus 5 billion years ago. Each shone 100 times brighter than the brightest regular supernova, and regular supernovae can themselves shine hundreds of times brighter than regular suns. If quark stars exist, they may hold about twice as much mass as our sun, but in an area only a few miles across. Our sun is 109 times larger than Earth, so the quark star would hold more mass than our sun in a thimble, compared to our sun's size. The stress of such confinement may cause neutrons to break down into quarks, the building block of matter. In theory a quark star could behave like one enormous subatomic particle, calling into question all manner of theories about the nature of matter, causation and space-time.

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Quark stars, exotic objects that have yet to be directly observed, are part of a new theory to explain some of the brightest stellar explosions recorded in the universe.
Super-luminous supernovae, which produce more than 100 times more light energy than normal supernovae and occur in about one out of every 1,000 supernovae explosions, have long baffled astrophysicists. The problem has been finding a source for all of that extra energy.
University of Calgary astrophysicists Denis Leahy and Rachid Ouyed think they have a possible source the explosive conversion of a neutron star into a quark star.

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