Quark stars

Abstract After some historical remarks we discuss different criteria of dynamical stability of stars and the properties of the critical states where the loss of dynamical stability leads to a collapse with formation of a neutron star or a black hole. At the end some observational and theoretical problems related to quark stars are discussed. 

Keywords stellar stability, white dwarfs, neutron stars, quark stars... 

The discovery of the quark structure of matter led to the suggestion of possible existence of quark stars, which are even more compact than neutron stars. In the presence of indefiniteness concerning the quark structure of matter it is not possible now to make definite statements about the existence or nonexistence of stable quark stars, observational and theoretical investigations on this topic are still in progress. 

In this review I first make a historical excursus into the problem, mentioning the results of the key works. Several criteria of stability are discussed, with the main focus on the static criteria, and the energetic method, which permits to obtain conclusions about the stability (sometimes approximate) in a most simple way. Critical states of compact stars at the boundary of the dynamic stability are considered, at which the star is becoming unstable in the process of energy losses, and a collapse begins leading to formation of a neutron star or a black hole. Physical processes leading to a loss of stability are discussed. At the end some observations and theoretical problems connected with quark stars are considered. 

Larger scattering of the properties of quark stars is obtained by Andersen and Strickland (2002), who used the same Hard-Dense-Loop approach, but with wider variation of parameters. Their models of quark stars are represented in Fig. 10. 

Figure 10. Mass-radius relation for a quark star with A//u = 1.6 and A/n = 1. The weak-coupling results for the same choice of renormalization scales are shown as dashed lines, from Andersen and Strickland (2002).

Speculations about connection of the cosmological GRB with transition from the hadronic to quark star seems to be interesting, because they explain connection between GRB and supernovae explosion, with arbitrary time delay between these events, including very large, exceeding the Hubble time. 

Kohri, K., Iida, K., Sato, K. (2003). Upper limit on the mass of RX J1856.5-3754 as a possible quark star. Prog.Theor.Phys., 109 765-780. 

Turolla, R., Zane, S., Drake, J.J. (2003). Bare Quark Stars or Naked Neutron Stars The Case of RX J1856.5-3754. Astrophys. J.603 265-282. 

Here and below we will not distinguish between NSs, quark stars, hybrid stars etc. unless explicitely stated. 

Bare neutron and quark star emission (Turolla et al. 2004). 

Cooling curves for quark stars (see contributions by Grigorian et al. and others in this procedings)... 

In connection with quark stars one can speculate, that additional energy due to deconfinement can lead to additional kick, so that among high velocity compact objects the fraction of quark stars can be higher. For example, if the... 

Astronomy is in some sense a unique science we have only emission from objects under investigation. Because of that there is a wide field for speculations. Having a lot of uncertain parameters to explain properties of observed compact objects we have troubles even without quark stars ... 

What do we want to have from theoretical physicists is an input for astro-physical models of compact objects to produce some output which can then be compared with observations. We want initial parameters of quark stars + evolutionary laws ... 

Figure 8. The mass-radius relation, for the linear equation of state (12), of a static quark star.

Nowadays it is widely accepted that there should be realized various phases of QCD in temperature (T) - density (ftp,) plane. When we emphasize the low T and high pp region, the subjects are sometimes called physics of high-density QCD. The main purposes in this field should be to figure out the properties of phase transitions and new phases, and to extract their symmetry breaking pattern and low-energy excitation modes there on the basis of QCD. On the other hand, these studies have phenomenological implications on relativistic heavy-ion collisions and compact stars like neutron stars or quark stars. 

Neutron and quark stars are natural laboratories to investigate the interplay of strong, electro-weak and gravitational interaction. Many theoretically determined properties of these astrophysical objects were tested by the observed properties of pulsars, and detailed calculations exist for these stars[ 1 —4. ... 

DIQUARK CONDENSATION EFFECTS ON HOT QUARK STAR CONFIGURATIONS... 

Diquarks gained popularity in astrophysics about a decade ago, when they were suggested to influence the supernova collapse and bounce-off [8-12], and to enhance the neutrino cooling of quark-stars. The latter effect is now subject to much research within improved scenarios Refs. [7, 9, 8]. 

Diquark condensation effects on hot quark star configurations... 

Here we compare configurations with and without CS, in order to investigate the effect of diquark condensation on the total energy of a quark star and to... 

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