Bag Model Parameters Research Articles

The possibility of twin star solutions in a model based on lattice QCD thermodynamics

The properties of compact stars and in particular the existence of twin star solutions are investigated within an effective model that is constrained by lattice QCD thermodynamics. The model is modified at large baryon densities to incorporate a large variety of scenarios of first order phase transitions to a phase of deconfined quarks. This is achieved by matching two different variants of the bag model equation of state, in order to estimate the role of the Bag model parameters on the appearance of a second family of neutron stars. The produced sequences of neutron stars are compared with modern constrains on stellar masses, radii, and tidal deformability from astrophysical observations and gravitational wave analyses. It is found that those scenarios in our analysis, in which a third family of stars appeared due to the deconfinement transition, are disfavored from astrophysical constraints.

Hybrid Stars that Masquerade as Neutron Stars

We show that a hybrid (nuclear + quark matter) star can have a mass-radius relationship very similar to that predicted for a star made of purely nucleonic matter. We show this for a generic parameterization of the quark matter equation of state, and also for an MIT bag model, each including a phenomenological correction based on gluonic corrections to the equation of state. We obtain hybrid stars as heavy as 2 M_solar for reasonable values of the bag model parameters. For nuclear matter, we use the equation of state calculated by Akmal, Pandharipande, and Ravenhall using many-body techniques. Both mixed and homogeneous phases of nuclear and quark matter are considered.

Models of Strange Stars with a Crust and Strange Dwarfs

Strange quark stars with a crust and strange dwarfs consisting of a compact strange quark core and an extended crust are investigated in terms of a bag model. The crust, which consists of atomic nuclei and degenerate electrons, has a limiting density of ρcr=ρdrip=4.3·1011 g/cm3. A series of configurations are calculated for two sets of bag model parameters and three different values of ρcr (109 g/cm3 ≤ ρcr ≤ ρdrip) to find the dependence of a star's mass M and radius R on the central density. Sequences of stars ranging from compact strange stars to extended strange dwarfs are constructed out of strange quark matter with a crust. The effect of the bag model parameters and limiting crust density ρcr on the parameters of the strange stars and strange dwarfs is examined. The strange dwarfs are compared with ordinary white dwarfs and observational differences between the two are pointed out.

Upper Limit on the Mass of RX J1856.5-3754 as a Possible Quark Star

Recent deep Chandra LETG+HRC-S observations suggest the possibility that RX J1856.5--3754 is a compact star whose radiation radius is 3.8--8.2 km. In this paper, we systematically calculate the mass-radius relations of quark stars within the bag model. Assuming that RX J1856.5--3754 is a pure quark star, we derive an upper limit on its mass for various sets of the bag-model parameters.

Possible Quark Star RX J1856.5-3754 and Its Mass

The possibility that RX J1856.5-3754 is a compact star whose radiation radius is 3.8-8.2 km was suggested by recent deep Chandra LETG+HRC-S observations. If we assume that RX J1856.5-3754 is a pure quark star, we obtain an upper limit on its mass for various sets of the bag-model parameters [K. Kohri, K. Iida and K. Sato, Prog. Theor. Phys. 109 (2003), 765].

Collective modes in a strangelet.

Possible existence of various collective modes, such as the compressional, the polarizational, and the surface vibrational modes in a strangelet, are discussed in the framework of hydrodynamic model. The eigenenergies of these collective modes are estimated using an equation of state for the interacting quark matter based on the quark bag model and are compared with the corresponding modes in the nuclei. It is found that, within the uncertainties associated with the bag model parameters, the collective eigenenergies in a strangelet are 1.5\char21{}4 times higher than those in the nuclei of same baryon number. The collective mode behavior may distinguish a strangelet from a large nucleus and give signature of the formation of quark-gluon plasma in ultrarelativistic heavy-ion collision experiments.

Maximum rotation frequency of strange stars.

Using the MIT bag model of strange-quark matter, we calculate the maximum angular frequency of the uniform rotation of strange stars. After studying a broad range of the MIT bag-model parameters, we obtain an upper bound of 12.3 kHz.

Chiral-quark-gluon description of the nuclear force.

We discuss the restoration of the chiral symmetry in the case of the N-N interaction on the basis of the cloudy bag model. A quantum-chromodynamics-based many-body theory of confined quarks, confined gluons, and pions is developed. In terms of the Green's functions and vertex functions of these fields, the energy shift due to gluon and pion exchange between quarks is calculated systematically and the short-range and long-range parts of the effective nuclear potential are obtained. For the short-range part, a soft repulsive core is obtained using the cloudy bag model parameters and the main contribution comes from the gluon exchange, as expected. The height of the core, however, is sensitive to the (poorly determined) subtraction required to remove the kinetic energy of the relative N-N motion. For the long-range part, an explicit formula, which is the same as one-pion-exchange potential, is obtained, but the strength of this part is 30% lower than that of one-pion-exchange potential when ${f}_{\ensuremath{\pi}}$ is chosen as 93.0 MeV.

One-gluon exchange and the nuclear strong interaction

Properties of nuclear matter are calculated with a quark model in the spirit of the MIT bag model. A many-body wave function is written in terms of the quark degrees of freedom. Through this wave function local correlations are put in the many-quark system such that three quarks close to each other are dominantly in a color singlet nucleon state. There is a range of bag model parameters for which the energy per nucleon of this correlated state is lower than that of a quark Fermi gas near normal nuclear matter density.