Abstract
We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different compact star solutions: traditional neutron stars, strange quark stars, as well as two non-standard solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the stationary properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo. The assumptions underlying the viability of the different star types, in particular those corresponding to absolutely stable quark matter, are finally discussed at some length.
Highlights
Vacuum, i.e., that it has a lower energy per baryon ratio at zero pressure than quark matter
The models that are typically considered in this context are dual to theories that differ from Quantum Chromodynamics (QCD) in a number of ways, and are studied in their large-N and infinitely strongly coupled limits
While the holographic dual of QCD is still unknown, the strongly coupled regime of this theory covers practically all energies of phenomenological interest, including in particular the densities realized inside compact stars
Summary
As supersymmetry is broken by the chemical potential, states with a large density could exhibit similar properties in the sectors that the two theories share, even though the theories have very different vacua. As discussed above, this is what has been seen to happen at finite temperature and zero density. Using the holographic dual description, it can be shown that the renormalized mass coincides with the energy gap between the vacuum and a state with a quark For this reason, when matching to QCD, it is natural to consider the mass in the holographic model as closely related to the constituent quark mass, rather than to the bare quark mass. The virtue of our model is, that it is the simplest and best studied holographic model, and, as we will show, it fares no worse than any other existing model when confronted with observations
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