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

¹ Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
² INFN Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
³ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Bât. B5a, 4000 Liège, Belgium
e-mail: prasanta.char@uliege.be

Received: 14 June 2021
Accepted: 9 January 2022

Abstract

The existence of massive compact stars (M ≳ 2.1 M_⊙) implies that the speed of sound exceeds the conformal limit (c_s² = 1/3 × the squared speed of light in vacuum) if those stars have an inner and outer crust of ordinary nuclear matter. Here, we show that if the most massive objects are strange quark stars, namely, stars entirely composed of quarks, c_s can assume values below the conformal limit even 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 use a Bayesian analysis framework and by adopting a constant speed of sound equation of state to show that the posterior distribution of c_s² is peaked around 0.3 and the maximum mass of the most probable equation of state is ∼2.13 M_⊙. We discuss which new data would require a speed of sound larger than 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 2.5 M_⊙, as it would be if the secondary component of GW190814 would turn out to be a compact star – and not a black hole, as previously assumed. Finally, we discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration compare with our results and find they are in qualitative agreement. We conclude with a brief discussion of other possible interpretations of our analysis.