The effect of dynamical quark mass in the calculation of strange quark star structure
Abstract
We have discussed dynamical behavior of strange quark matter components, in particular the effects of density dependent quark mass on the equation of state of strange quark matter. Dynamical masses of quarks have been computed within NambuJonaLasinio (NJL) model, then we have done the strange quark matter calculations employing the MIT bag model with these dynamical masses. For the sake of comparing dynamical mass interaction with QCD quarkquark interaction, we have considered the onegluonexchange term as the effective interaction between quarks for MIT bag model. Our dynamical approach illustrates an improvement for the obtained values of equation of state. We have also investigated the structure of strange quark star using TolmanOppenheimerVolkoff (TOV) equations for all applied models. Our results show that the dynamical mass interaction leads to lower values for the gravitational mass.
I Introduction
Strange quark stars (SQS) are the most compact objects with a surface density , which is about fourteen orders of magnitude greater than the surface density of neutron stars, while their central density could be up to five times higher than that (Haensel et al. Haensel2007 ; Glendenning Glendenning2000 ; Weber Weber1999 ). It was first Itoh (1970) that, even before QCD full development, proposed SQSs which is made of strange quark matter (SQM). Later, Bodmer (1971) discussed the fate of an astronomical object collapsing to such a state of matter.
The quark deconfinement hypothesis is one of the exciting steps in investigation for the building blocks of matter. Soon after predictions of quarks in theories and successful laboratory observations, many hadronic models were developed to describe the probable quark matter proposed at high energy regimes. In the 1970s, after formulation of QCD, perturbative calculations of the equations of state of SQM got form, but the area of validity for these calculations was restricted to very high densities (Collins & Perry Collins1975 ). The existence of SQSs was also discussed by Witten (1984), who conjectured that a first order QCD phase transition in the early universe could concentrate most of the quark excess in dense quark nuggets. Witten proposed that SQM composed of light quarks is more stable than nuclei, therefore SQM can be considered as the ground state of matter.
An SQS would be the bulk SQM phase consisting of almost equal numbers of up, down and strange quarks, plus a small number of electrons to ensure charge neutrality. A typical electron fraction is less than and decreases from the surface to the center of an SQS (Haensel et al. Haensel2007 ; Glendenning Glendenning2000 ; Weber Weber1999 ; Camenzind Camenzind2007 ). SQM would have a lower chargetobaryon ratio compared to the nuclear matter and can show itself in the form of an SQS (Witten Witten1984 ; Alcock et al. Alcock1986 ; Haensel et al. Haensel1986 ; Kettner et al. Kettner1995 ). The collapse of a massive star could lead to the formation of an SQS. An SQS may also be formed from a neutron star and is denser than the neutron star (Bhattacharyya et al. Bhattacharyya2006 ). If sufficient additional matter is added to an SQS, it will collapse into a black hole. Neutron stars with masses of with rapid spins are theoretically the best candidates for conversion to an SQS. An extrapolation based on this indicates that up to two quarknovae occur in the observable universe each day. In addition, recent Chandra observations indicate that objects and may contain SQSs (Prakash et al. Prakash2003 ). Other investigations also show that the object may be an SQS (Yu & Xu Yu2010 ).
The strange quark star, founded from quark matter theory consists of too many unsolved puzzles which are usually involved in physics of these relativistic objects. System complexity of these stars prohibit us from considering all physical and astrophysical properties simultaneously, and it is possible that some parameters entering the equation of state do not represent specific physical properties. For example, in MIT bag model, one of the models used in this paper, when the researchers try to find and fit the bag constant according to informations achieved from big colliders (Jin & Jenning Jin1997 ; Alford et al. Alford1998 ; Blaschke et al. Blaschke1999 ; Burgio et al. Burgio2002b ; Begun et al. Begun2011 ), we should keep this principle as a matter of fact that different parameters like temperature, electromagnetic intensity, density etc. are important enough on final interpretation for theoretical calculated bag constant. In this point of view even constant values of bag pressure no more can be considered purely as the energy density difference between the perturbative vacuum and the true vacuum. The role of bag constant for confining quark matter in comparison with gravity confinement for neutron matter may require more attention when we consider it for compact stars. Therefore it is better to consider the dynamical properties of the parameters for investigating of the properties of quarks. Many works have been done to adapt the theory of bag Model on physics of ultradense matter like using a density dependent bag constant (Burgio et al. Burgio2002a ), utilizing different values of coupling constants for one gluon exchange (Farhi & Jaffe Farhi1984 ; Berger & Jaffe Berger1987 ), or instead considering dynamical mass as effective interaction between particles (Peng et al. Peng1999 ; Shao et al. Shao2011 ).
From perturbative QCD, we know that quarks at ultra high densities asymptotically interact. One way of considering the interaction is to assume that quarks exchange one gluon. Therefor we can add a term to equation of state that is characterized by a coupling constant. But constant values of this parameter will weaken the power of interaction in lower densities while in higher densities, it increases it. One method to solve the problem is to assume a density dependent quark mass as the effective interaction. This approach was investigated in references (Fowler et al. Fowler1981 ; Chakrabarty et al. Chakrabarty1989 ; Chakrabarty Chakrabarty1991 , Chakrabarty1994 ; Benvenuto & Lugones Benvenuto1995 ; Lugones & Benvenuto Lugones1995 ). This has been done by adding a term to the rest mass which is characterized by a free parameter determined by stability conditions. They concluded that the density dependent mass is flavor independent and the applied free parameter has the same meaning as the bag constant. Then by selecting one value of bag constant for all densities and flavors, they tried to obtain the equation of state of quark matter (Peng et al. Peng1999 ). A better approach closer to current work, is to find a solution for density dependent mass from NambuJonaLasinio (NJL) method (Carol Carol2009 ). Carol calculated the equation of state and structure for hybrid stars within MIT bag model, while numerical values of density dependent mass entering in the energy equation had been obtained from dynamical calculations of mass in NJL model. These numerical values were entered directly in the pressure equation without considering density dependency. Quark masses and NJL constants were also approximate values. The bag constant in that work was density independent; therefore in addition to previous known problems of constant values for this parameter (Baldo et al. Baldo2006 ; Alford & Reddy Alford2003 ; Alford et al. Alford2005 ), it misinterprets the meaning of the effective interaction in some densities.
In Our previous work we considered a hot strange star just after the collapse of a supernova (Bordbar et al. Bordbar2011vol54 ), at finite temperature with a density dependent bag constant. The calculations for the structure properties of the strange star at different temperature indicates that it’s maximum mass decreases by increasing the temperature. In another work (Bordbar & Peivand Bordbar2011vol11 ), we concentrated on the calculation of a bulk of spin polarized SQM at zero temperature in the presence of a strong magnetic field. We computed structure properties of this system and found that the presence of a magnetic field leads to a more stable SQS when compared to the structure properties of an unpolarized SQS. In present paper, we investigate the quark matter equation of state and the strange quark star structure following Carroll (Carol Carol2009 ). We base our calculations on MIT bag model, and after following NJL formalism we extrapolate a density dependent equation from numerical values of dynamical mass obtained using NJL method. In Sec. II, the required equations for the MIT bag model are written, the same has been done for NJL model. In Sec. II.3, we describe the formalism applied in this article, and after solving TOV equations in Sec. III, we calculates SQS structure for our method.
Ii Calculation of equation of state for SQM
In this section, we calculate the equation of state of strange quark matter (SQM) using MIT and NJL methods as well as MIT method with the dynamical mass. At first, we introduce these three models in three separate sections, then we give our results for the energy and the equation of state of SQM in sec. II.4.
ii.1 The MIT Bag Model
Total energy of a bulk of deconfined up (), down () and strange () quarks within MIT bag model is as follows (Witten Witten1984 ; Farhi & Jaffe Farhi1984 ; Baym Baym1985 ; Baym et al. Baym1985 ; Berger & Jaffe Berger1987 ; Glendenning Glendenning1990 ; Maruyama et al. Maruyama2007 ):
(1) 
In Eq. (1), is the bag constant, and
(2) 
where denotes the flavor of the relevant quark, is QCD coupling constant and the following term demonstrates the onegluonexchange interaction. In above equation, is defined as follows,
(3) 
where the Fermi momentum is given by
(4) 
For the bag constant (), we use a density dependent Gaussian parametrization (Burgio et al. Burgio2002a ; Baldo et al. Baldo2006 ):
(5) 
with and . In SQM, the betaequilibrium and charge neutrality conditions lead to the following relation for the number density of quarks,
(6) 
From the total energy, we can obtain the equation of state of SQM using the following relation,
(7) 
ii.2 The NambuJonaLasinio Model
Here we give a brief introduction regarding the calculations in the NambuJonaLasinio (NJL) method. For NJL model, we use a common three flavor lagrangian adopted from (Rehberg et al. Rehberg1996 ) which preserves chiral symmetry of QCD,
(8)  
In adopted lagrangian, denotes quark field with three flavors , and , and three colors. is a matrix in flavor space. And ( ) are the flavor matrices. We restrict ourselves to the isospinsymmetric case, . We have picked up the parameters from references (Kunihiro Kunihiro1989 ; Ruivo et al. Ruivo1999 ; Buballa & Oertel Buballa1999 ) which are fitted to the pion mass, the pion decay constant, the kaon mass and the quark condensates.
NJL model is an unrenormalizable method with divergent integrations. To prevent the divergence, we need to introduce some breaking points for upper limit of integrals which satisfy the physics ranges of our problem. It is usually done by choosing a proper cutoff. In present paper, the adopted cutoff is named Ultraviolet cutoff that indicates restoring of chiral symmetry breaking, . and are coupling strengths that read, . The rest mass of quark is , and there is for and quarks. The baryon number density is given by
(9) 
where . Within mean field approximation, the dynamical mass is calculated by the following gap equation,
(10) 
In the above equation, we need to calculate permutation of all quark flavors. The quark condensate in Eq. (10) reads
(11) 
and , Fermi momentum of quark , is obtained from the following relation,
(12) 
Equations (10) and (11) have self consistent solutions. It means that for a given number density, , we should calculate quark condensate and substituting the corresponding value in Eq. (10) to reach a consistent result of the dynamical mass after doing the iteration process. In Fig. 1, we have plotted the results of density dependent mass for , and quarks as a function of density. As it is clear from Fig. 1, quark masses vary from current masses ( for and quarks, and for quark) at high densities to constituent mass at near zero densities ( for and quarks, and for quark).
The solution via mean field approximation forces us to stabilize equations by diminishing energy density and pressure in vacuum. This is satisfied by entering a parameter which has the same meaning of bag constant in MIT bag model (Buballa & Oertel Buballa1999 ):
(13) 
Now we can calculate the equation of state of SQM in NJL model,
(14) 
where
(15) 
Parameter is the bag pressure, which is explained by Buballa (2005), and is a dynamical consequence of the mean field solution, not a parameter inserted by hand, as was done in MIT bag model. It is shown in Fig. 1, matter in NJL method acquires dynamical mass in nonzero baryon densities, but in MIT bag model, the given mass remains constant for all densities. Consequently, this will lead to dissimilar chiral symmetry behavior as density changes. In NJL model, since quarks acquire dynamical mass, the chiral symmetry spontaneously breaks in lower densities, while in MIT bag model, it will happen physically when quarks change their directions by hitting the bag (what is not considered theoretically in ordinary MIT bag model). The bag constant versus density is presented in Fig. 2 for our used models. It is apparent from Fig. 2 that chiral symmetry in our calculations is fully restored in the densities greater than . It is also important to mention that vacuum in MIT bag model is totally free of particles (flow of particle’s wave function is restricted by the confinement), while in NJL model no confinement is produced. In other word, the vacuum in NJL model is made of paired quasiquarks that lower the energy density of particles in comparison to MIT bag model. From the above discussions, it seems reasonable to add an effective bag constant to energy equation (Buballa Buballa2005 ),
(16) 
From Fig. 2, it seems that the effective bag constant diminishes at zero density. Then the correct interpretation for the effective bag constant is the energy per volume needed to fully break quarkantiquark pairs in order to completely restore chiral symmetry at ultra high densities. Even the maximum value of dynamical NJL bag constant is smaller than that of MIT’s one, because it reduces the energy per particle due to quarkantiquark pairing at lower densities (Buballa Buballa2005 ). Fig. 2 shows that the decreasing rate of MIT bag constant is higher than that of NJL. This indicates that MIT bag model does gross approximation over physics of matter in middle and higher densities . Therefore, the density dependent bag constant should be corrected by another higher density sensitive parameter. This could not be achieved by a one gluon exchange term that considers the interaction with a constant strength in all energy regimes. Fig. 2 indicates that at the density , there is a cross point for the effective bag constant of NJL model and the bag constant of MIT model. As it is mentioned in above discussions, the bag pressure is the energy needed to confine particles where effective bag constant is energy needed to destabilize quarkantiquark pairs. Now, we can suggest that the hadronquark phase transition can takes place at the density . This is in good agreement with the results of others (Heinz Heinz2001 ; Heinz & Jacob Heinz2000 ).
ii.3 MIT bag model with dynamical mass
In MIT bag model with dynamical mass, we consider the effect of dynamical behavior of the quark mass in calculating the equation of state of SQM within MIT bag model using NJL numerical mass results. In fact, we use the dynamical masses (Fig. 1) for , and quarks in Eq. (II.1) instead of their fixed values.
ii.4 Our results for the energy and equation of state of SQM
To distinguish numerous outcomes, we present the results of our calculations in three following models;

Model 1: MIT model by a density dependent bag constant and one gluonexchange as effective interaction.

Model 2: NJL model.

Model 3: MIT bag model by a density dependent bag constant, dynamical mass and one gluonexchange as effective interaction.
Our results for the energy of SQM versus density calculated with above models have been plotted in Fig. 3. We see that for both MIT based calculations (models 1 and 3), at lower densities , the energy of SQM suddenly increases as the density decreases. This shows the concept of confinement (Buballa Buballa2005 ). For these two models, we also see that the energy of SQM gets to a minimum, then increases with a small rate. Fig. 3 shows that for model 1 and model 3, the energies of different coupling constants are nearly identical for densities . However, they have a substantial difference as the density increases. We can see that at lower densities , the results of model 3 is considerably different from those of model 1. While this difference becomes small as density increases, specially for lower values of coupling constant, due to asymptotic freedoms which is the simple MIT bag model without interaction. From Fig. 3, it is seen that the energy of SQM in model 2 (NJL model) has finite values even at low densities showing no confinement. We also see that the energy of SQM from model 3 with smaller values of coupling constant is lower than that of model 2 for indicating a more stable state of quark matter at these densities. However, at very high densities, the difference between the results of these two models becomes negligible.
In Fig. 4, our results for the pressure of SQM have been plotted versus density. It can be found that for MIT bag model, the higher values of coupling constant leads to the stiffer equation of state for SQM. Fig. 4 shows that by considering a dynamical mass for the quarks (density dependent mass) in MIT model, we get the lower values for the pressure of SQM. For , we see that the result of model 3 for the equation of state of SQM is nearly identical with that of model 1. It can be seen that for , our results for the pressure of SQM calculated by NJL model are nearly identical with those of model 3 and model 1 for , while at lower densities, there is a considerable difference between them.
In order to investigate the quark matter stability, the energy of SQM versus pressure has been plotted in Fig. 5. It is clearly seen that at zero pressure, the MIT bag model with leads to the lowest value for the energy of SQM () compared to other models. This value is comparable with the result for the binding energy per particle of () (Witten Witten1984 ). This indicates that among different models used in this work, MIT model with shows the most stable state of SQM.
Iii Calculation of strange quark star structure
The gravitational mass () and radius () of compact stars are of special interest in astrophysics. In this section, we calculate the structural properties of a strange quark star for our three models. Using the equation of state of strange quark matter for the models applied in this work, we can obtain and by numerically integrating the general relativistic equations of hydrostatic equilibrium, the TolmanOppenheimerVolkoff (TOV) equations, which are as follows (Shapiro & Teukolsky Shapiro1983 ),
(17) 
(18) 
where is the energy density, is the gravitational constant, and
(19) 
has the interpretation of the mass inside radius . By selecting a central energy density , under the boundary conditions and m(0)=0, we integrate the TOV equation outwards to a radius , at which vanishes.
In Fig. 6, we have presented our results for the gravitational mass of SQS versus the central energy density. Fig. 6 shows that at low energy densities, the gravitational mass increases rapidly by increasing the energy density, and it finally reaches to a limiting value (maximum gravitational mass) at higher energy densities. It is seen that the increasing rate of mass for Model 3 with higher values of coupling constant is substantially higher than those of other models. Table 1 summarizes maximum gravitational masses of different applied models and the corresponding radii. As it seen from Table 1, we can conclude that using dynamical mass in energy equation and equation of state of SQM reduces the calculated maximum mass. This is in a good agreement with many observational data obtained from low mass compact stars (Zhang Zhang2007 ). It is interesting that in spite of considering dynamical mass as the effective interaction in MIT bag model (model 3 with ), we find the smaller SQS maximum mass in comparison to MIT bag model (model 1) even without interaction (). As it is obvious from Table 1, for models 1 and 3, the calculated maximum mass increases as strong coupling constant increases. This behavior demonstrates that ultra massive SQS with masses greater than are stars which are composed of highly interacting strange quark matter. We note that some studies indicate that there exist a big uncertainty about mass and radius of ultra massive stars with (Lattimer & Prakash Lattimer2010 ). These studies showed that the observed data of mass and radius for these stars, which commonly belong to Xray stars, were wrongly calculated and the calculations were revised to the smaller values for mass and radius. The best example is pulsar that initially was supposed to have a mass of but recently revised to (Lattimer & Prakash Lattimer2010 ).
We have also plotted the gravitational mass of SQS versus radius for our three models in Fig. 7. It is seen that for all models, the mass increases by increasing the radius, but with different increasing rates for different models. Fig. 7 shows that for a given value of radius, the dynamical model (model 3) gives the smaller mass with respect to that of MIT bag model (model 1); however, for , it is close to the result of NJL model (model 2).
Acknowledgements
This work has been supported by Research Institute for Astronomy and Astrophysics of Maragha. We wish to thank Shiraz University Research Council.
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Model 1; 
1.43  7.61 
Model 1;  1.73  8.17 
Model 1;  2.6  10.6 
Model 2  0.98  5.59 
Model 3;  1.05  6.03 
Model 3;  1.65  6.98 
Model 3;  2.3  8.69 