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

TL;DR

This paper investigates whether the speed of sound in dense matter within compact stars can respect the conformal limit while still explaining massive stars, using Bayesian analysis of astrophysical data and considering strange quark stars.

Contribution

It demonstrates that strange quark stars can satisfy the conformal limit of the speed of sound while matching observational data, challenging previous assumptions about nuclear matter in massive stars.

Findings

Posterior distribution of c_s^2 peaks around 0.3

Maximum mass of most probable EOS is ~2.13 M_sun

Strange quark stars can explain massive stars without violating the conformal limit

Abstract

The existence of massive compact stars $(M\gtrsim 2.1 M_{\odot})$ 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 $\sim 2.13 M_{\odot}$. 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.