Solid Bare Strange Quark Stars

The reason, we need three terms of ‘strange’, ‘bare’, and ‘solid’ before quark stars, is presented concisely though some fundamental issues are not certain. Observations favoring these stars are introduced. 1. Hadronic stability: why strange? Strange (quark) stars are quark stars with strangeness. What’s strangeness? Some peculiar cosmic ray events were discovered in 1947, with a strange nature of “being produced rapidly in pairs, but decaying slowly independently”. These particles were thus called “strange” particles, and a new quantum number, “strangeness”(S), was introduced. In the standard model of particle physics, it is known that the strangeness represents actually the existence of a new kind of quarks, the strange quark, s. In this model, a neutron is composed of three valence quarks (one up and two down quarks), sea quark pairs (uū, dū, ss̄), and gluons, i.e., n = {udd, uūdd̄ss̄, g}; a proton p = {uud, uūdd̄ss̄, g}. A neutron (or proton) does not have strangeness because of S(s) = −1 and S(s̄) = 1. Hadrons with three valence quarks are called baryons (e.g., protons and neutrons). And then, how to produce strangeness in a star if it is the remains of an evolved main sequent star? Atomic nuclei in normal stars are made only of the two nucleons (proton and neutron), without strangeness. Two scenarios are outlined for creating strangeness in dense stellar matter. (1) The first one is in the hadronic degrees of freedom (i.e., hadrons as quasiparticles). Hyperons are baryons with strangeness, in which one or more valence quarks are replaced by s-quarks (e.g., Λ = {usd}, Ω = {sss}). Nucleons with high enough Fermi energy (e.g., in traditional neutron stars) may decay into hyperons by weak interactions. Such nuclear matter with strangeness is called as strange hadronic matter, and the corresponding neutron stars are named as hyperon stars. The study of hypernuclei may help us to understand the hyperon stars. (2) The second one is in the quark degrees. Suppose that quarks within hadrons are deconfined in high density (to form quark-gluon plasma, or called quark matter), one may expect that 2-flavor quark matter (i.e., u and d) appears in “neutron” stars. However, in case of s-quark mass being smaller than the Fermi energy of uand d-quarks, the system may become more stable via weak-interaction, decaying into 3-flavor quark matter. A further radical speculation is that bulk 3-flavor quark matter is absolutely stable (the Bodmer-Witten’s conjecture); strange stars (e.g., Xu 2002a) are accordingly made of such strange quark matter (SQM) with nearly equal numbers of the light quarks. Note: an SQM core and outer nuclear matter may coexist over macroscopic scale if bulk SQM is metastable (mixed stars). And then, can hadrons (e.g., neutrons) in “neutron” stars be deconfined? The underlying theory of the interaction between quarks is believed to be quan-