Quark Stars: From Quark-Gluon Plasma to Supernova Remnants
Neutron stars may contain quark-gluon plasma in their cores—a state of matter where protons and neutrons disintegrate into quarks and gluons. These objects are about 20 km in diameter, with a mass of 1.4 times that of the Sun, and their density allows quarks to exist freely under the pressure of the strong interaction. The hypothesis suggests a transition from a neutron crust to a quark core, where matter is more stable than ordinary hadrons.
Quarks are fermions inside protons and neutrons, held together by gluons. In quantum chromodynamics, quark-gluon plasma forms a perfect fluid with zero viscosity, as seen in experiments at CERN and Brookhaven. In stars, this is a cold, ultra-dense form that dominates over gravity.
The Physics of Quarks and the Strong Interaction
Atoms consist of nuclei (protons, neutrons) and electrons. Each nucleon is made of three quarks: up (u), down (d), and sometimes strange (s). Fermions obey the Pauli exclusion principle, preventing identical states, unlike boson gluons.
Strange quarks have a long lifetime, decaying into u and d. Quark-gluon soup was the basis of the early universe after the Big Bang, transitioning into hadrons. In neutron stars, deconfinement turns neutrons into free quarks just beneath the crust.
Structure of Neutron Stars
Neutron stars are supernova remnants, pulsars with periodic radio pulses. Their density causes neutron disintegration: the strong interaction prevails, forming cold, ultra-dense matter of unknown composition.
Measuring mass and radius is challenging due to distances (the nearest is 400 light-years away). Collisions generate gravitational waves, revealing the viscosity of matter. Bulk viscosity is estimated through perturbation theory: it characterizes energy loss during oscillations of the quark mixture density.
- Mass and Radius: Statistics from mergers show variations, indicating a possible quark core.
- Viscosity: Low, similar to plasma from accelerators.
- Pulsar Timing: A method for precise parameters.
- Gravitational Waves: Data from LIGO/Virgo on matter mixing.
The crust layer has millimeter-scale irregularities, while the core may be a quark soup.
The Quark Star Hypothesis
Quark stars are entirely composed of free quarks (u, d, s), called strange stars. Deconfinement occurs at a critical mass when the pressure of degenerate neutron gas is insufficient.
Strange matter is stable at zero pressure, the true ground state. Strangelets are quark droplets capable of transforming neutron stars.
Properties:
- Denser than neutron stars.
- Covered by a thin neutron crust.
- Low surface tension allows for macroscopic bodies.
- Potential for superconductivity via BCS theory.
- Energy comparable to thermonuclear fusion.
Simulations at JINR, CERN, and the Tokyo Institute: Bose-Einstein condensate transitioning to a superfluid solid phase.
Candidates Among Compact Objects
Fast-rotating pulsars and low-mass remnants are priorities. XTE J1739-285: 1122 Hz, radius 9–12 km, mass 1.2 M☉, 13,000 light-years away, in Ophiuchus. Possibly a pure quark structure under a gaseous envelope.
HESS J1731-347 (2022, Tübingen): A supernova remnant, 10,000 light-years away, mass 0.77 M☉, radius 10.4 km. Its X-ray spectrum is anomalous for a neutron star.
Key Takeaways
- Quark-gluon plasma in neutron star cores is a cold version of laboratory experiments.
- Collisions and gravitational waves provide data on viscosity and the equation of state.
- Candidates: XTE J1739-285 and HESS J1731-347 with extreme parameters.
- Strange matter is a stable state that could dominate compact objects.
- Future observations by SKAO will clarify the composition of stellar interiors.
— Editorial Team
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