Do the heaviest neutron stars have quark-matter cores?
In plain words
At the centre of the heaviest neutron stars, protons and neutrons may dissolve into free quarks. Whether this happens is not known.
Precise statement
For a neutron star near $M_{\mathrm{TOV}}$, determine whether the central density exceeds the onset of deconfined quark matter, diagnosed by the polytropic index $\gamma=d\ln P/d\ln\epsilon$ approaching its conformal value 1 and the normalized trace anomaly $(\epsilon-3P)/(3\epsilon)$ approaching 0, where $\epsilon$ is energy density. Answer: yes or no, with posterior probability given all astrophysical data and perturbative QCD limits.
What would settle it
Precise radii for two-solar-mass stars together with post-merger gravitational-wave frequencies sensitive to a phase transition.
Status in the literature
Annala et al. (Nature Physics 2020; Nature Communications 2023) inferred near-conformal matter in maximal-mass cores with high probability, but the conclusion depends on how the hadronic and quark phases are defined.
See also
- Related Does the sound speed exceed the conformal bound inside neutron stars?
- Related Location and origin of the sound-speed maximum in dense matter
- Related Existence and location of the QCD critical point
- Related Is the Cassiopeia A neutron star cooling unusually fast?
- Related How do hyperons fit inside two-solar-mass neutron stars?