Strangeon Stars

Abstract

Stable micro-nucleus is 2-flavored (u and d), whereas stable macro-nucleus could be 3-flavored (u, d and s) if the light flavor symmetry restores there. Nucleons are the constituent of a nucleus, while strangeons are named as the constituent of 3-flavored baryonic matter. Gravity-compressed baryonic object created after core-collapse supernova could be strangeon star if the energy scale (∼ 0.5 GeV) cannot be high enough for quark deconfinement and if there occurs 3-flavor symmetry restoration. Strangeon stars are explained here, including their formation and manifestation/identification. Much work, coupled with effective micro-model of strangeon matter, is needed to take advantage of the unique opportunities advanced facilities will provide. KEYWORDS: pulsar: general, dense matter, equation of stateAlthough the state equation of dense matter around nuclear density is a great challenge for physicists, it is usually a thought that strangeness could be relevant to the solution. As noted in the QC-S2014 proceedings [1], such kind of strange matter is conjectured to be strangeon matter, being manifested in the form of compact stars, cosmic rays, and even dark matter. In this contribution, we are focusing on strangeon star, which is a key ingredient of strangeon matter in astrophysics.It is half a century since the first pulsar was discovered, but the real nature of pulsar remains one of the most puzzling problems both in physics and in astronomy. Pulsars are always thought to be neutron stars, but the issue of their real structure is still controversial. Different observations (e.g., the spin period, the measurements of mass and radius) show that the typical density of pulsar-like compact stars could be only a few nuclear density, and the separation between quarks is ∼ 0.5 fm and hence the energy scale is order of ∼ 0.5 GeV according to Heisenberg's uncertainty relation. The pulsar structure is then a problem of non-perturbative QCD (quantum chromo-dynamics), an unsolved issue for fundamental strong interaction at low energy regime ( 1 GeV). Similar to the issue of turbulence, the strong interaction physics at low-energy is still challenging though QCD is mathematically well-defined. In fact, both of "Navier-Stokes Equation" and "Yang-Mills and Mass Gap" are prize problems named by the Clay Mathematical Institute. Hitherto now, no one can derive pulsar structure from ab-initio calculations, nonetheless astrophysicists could speculate phenomenologically.In Fig. 1, we summarize pulsar inner structures (i.e., models) speculators suggest. All of the pulsar structure models fell into four categories: hadron star, quark star, hybrid/mixed star and strangeon star, as are shown in Fig. 1. In the hadron star model, quarks are confined in hadrons, and the hadron liquid is the dominated part of the star. Hadron stars are bounded by gravity, and they must have crusts composed of ions and electrons. A hadron star is called a neutron star if only the nucleon (proton and neutron) degree of freedom is introduced. Conversely, a quark...

Strangeon Matter in a Liquid Drop Model

“…The Witten's conjecture [2] about dense matter could be extended to a generalized version: strange matter in bulk could be absolutely stable, in which quarks are either free (for strange quark matter) or localized (for strangeon matter) [1,3]. A strangeon may contain equal numbers of u, d and s quarks, and its number of quarks inside could be 6, 9, 12, 18 or even more.…”

“…A 3-flavored nucleus conjectured could be stable only if its baryon number is larger than a critical one, A c , in which strangeons are the constituent as an analogy of nucleons for nucleus [1]. We try to model strangeon matter in a sense of phenomenological liquid drop, with two free parameters: the mass per baryon of a strangeon in vacuum, M, and potential depth between strangeons, ϵ.…”