Speed of sound in dense matter and two families of compact stars

Abstract

The existence of massive compact stars $(M\gtrsim 2.1M_{\odot })$ implies that the conformal limit of the speed of sound $c_s^2=1/3$ is violated if those stars have a crust of ordinary nuclear matter. Here we show that, if the most massive objects are strange quark stars, i.e. stars entirely composed of quarks, the conformal limit can be respected while observational limits on those objects are also satisfied. By using astrophysical data associated with those massive stars, derived from electromagnetic and gravitational wave signals, we show, within a Bayesian analysis framework and by adopting a constant speed of sound equation of state, that the posterior distribution of $c_s^2$ is peaked around 0.3, and the maximum mass of the most probable equation of state is $\sim 2.13M_{\odot }$. We discuss which new data would require a violation of the conformal limit even when considering strange quark stars, in particular we analyze the possibility that the maximum mass of compact stars is larger than $2.5M_{\odot }$, as it would be if the secondary component of GW190814 is a compact star and not a black hole. Finally, we discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration compare with our analysis (not based on them) and with other possible interpretations.

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Cited by in corpus (11)

• Colour-Flavour Locked Quark Stars in Light of the Compact Object in HESS J1731-347 and the GW190814 Event
• Interacting $ud$ and $uds$ quark matter at finite densities and quark stars
• Insights on the peak in the speed of sound of ultradense matter
• Is the compact object associated with HESS J1731-347 a strange quark star?
• Implications of the Conformal Constraint on Sound Speed on the Radius of PSR J0952-0607 within Rastall Gravity
• Analyzing the speed of sound in neutron star with machine learning
• Impact of the nuclear equation of state on the formation of twin stars
• Properties of First-Order Hadron-Quark Phase Transition from Inverting Neutron Star Observables
• Equation of State of Decompressed Quark Matter, and Observational Signatures of Quark-Star Mergers
• How Neutron Star Observations Point Towards Exotic Matter: Existing Explanations and a Prospective Proposal
• Effects of a Brueckner-Hartree-Fock-corrected effective mass on speed of sound, conformality, and observables of dark matter-admixed neutron stars