A study of massive neutron stars with holographic multiquark cores

Abstract

Up to now, the state of matter inside a massive neutron star (NS) is still unclear. In the core of the massive NS, density and pressure become large until the matter inside could turn into deconfined matter e.g. quark-gluon plasma (QGP). Recently, results from a model-independent analysis [1] from a combination of perturbative theoretical calculations with observational data [2, 3] shows that quark matter could exist in the cores of massive NS with masses above 2.0 M. However, the QGP exists at a very high temperature. Compared to the relatively low temperature of the NS, the exotic quark matter might not be the free QGP but instead, the deconfined and chiral symmetry broken bound state called “multiquark’’. Due to the non-perturbative nature of the multiquark state, the usual description of the multiquark state as based on QCD faces many difficulties and becomes unreliable. In this thesis, we use a holographic QCD approach to describe the multiquark [4, 5] based on the Sakai-Sugimoto (SS) model and implement the equation of states (EoS) of multiquark matter to interpolate between the two known limits: the pQCD EoS at high densities, and the nuclear CET EoS at low densities. The multiquark EoS is relatively stiff at low densities and becomes softer at high densities. Thermodynamically, we found that the multiquark phase is more preferred over the extended CET nuclear matter above the transition points. From our study, the massive NS with a holographic multiquark core could have masses in the range between 1.96 − 2.23 (1.64 − 2.10)M. and radii between 14.3 − 11.8 (14.0 − 11.1) km for s = 26 (28) GeV fm−3 respectively. Effects of proton-baryon fractions are also studied for a certain type of baryonic EoS. We found that larger proton fractions could reduce the radius of the NS with the multiquark core by less than a kilometre. Additionally, we calculate the tidal Love number and dimensionless deformation parameter, k2 and, and they are found to be within the limit of the physically viable range under the present constraints. Finally, we calculate radial pulsation frequencies of the multiquark core for n = 0−5 modes for the entire mass range. As a result, for Mcore 2M!, the fundamental-mode frequency is around 2.5 kHz given that the energy density scale of the holographic SS model !s = 23.2037 GeV fm−3, this frequency is proportional to "!s.