Sophia Han

Constraining and applying a generic high-density equation of state

We discuss the “constant speed of sound” (CSS) parameterization of the equation of state of high density matter, and its application to the Field Correlator Method (FCM) model of quark matter. We show how observational constraints on the maximum mass and typical radius of neutron stars are expressed as constraints on the CSS parameters. We find that the observation of a 2 M star already severely constrains the CSS parameters, and is particularly difficult to accommodate if the squared speed of sound in the high density phase is assumed to be around 1=3 or less. We show that the FCM equation of state can be accurately represented by the CSS parameterization. We display the mapping between the FCM and CSS parameters, and see that FCM only allows equations of state in a restricted subspace of the CSS parameters. There are many models of matter at density significantly above nuclear saturation density, each with their own parameters. In studying the equation of state (EoS) of matter in this regime it is therefore useful to have a general parameterization of the EoS which can be used as a generic language for relating different models to each other and for expressing experimental constraints in model-independent terms. In this work we use the previously proposed “Constant Speed of Sound” (CSS) parameterization [1‐3]). We show how mass and radius observations can be expressed in terms of the CSS parameters. We then analyze a specific example, where the high-density matter is quark matter described by a model based on the Field Correlator Method (Sec. IV), showing how its parameters can be mapped on to the CSS parameter space, and how it is constrained by currently available observations of neutron stars. The CSS parameterization is applicable to high-density equations of state for which: (a) there is a sharp interface between nuclear matter and a high-density phase which we will call “quark matter”, even when (as in Sec. II) we do not make any assumptions about its physical nature; (b) the speed of sound in the high-density matter is pressure-independent for pressures ranging from the first-order transition pressure up to the maximum central pressure of neutron stars. One can then write the high-density EoS in terms of three parameters: the pressure ptrans of the transition, the discontinuity in energy density De at the transition, and the speed of sound c QM in the high-density phase. For a given nuclear matter EoS eNM(p), the full EoS is then e(p) = eNM(p)

Phase conversion dissipation in multicomponent compact stars

We propose a mechanism for the damping of density oscillations in multicomponent compact stars. The mechanism is the periodic conversion between different phases, i.e., the movement of the interface between them, induced by pressure oscillations in the star. The damping grows nonlinearly with the amplitude of the oscillation. We study in detail the case of r-modes in a hybrid star with a sharp interface, and we find that this mechanism is powerful enough to saturate the r-mode at very low saturation amplitude, of order

Constant-sound-speed parametrization for Nambu–Jona-Lasinio models of quark matter in hybrid stars

10^{-10}

Generic Conditions for Stable Hybrid Stars

, and is therefore likely to be the dominant r-mode saturation mechanism in hybrid stars with a sharp interface.

Color superconductivity in compact stellar hybrid configurations

The discovery of pulsars as heavy as 2 solar masses has led astrophysicists to rethink the core compositions of neutron stars, ruling out many models for the nuclear equations of state (EoS). We explore the hybrid stars that occur when hadronic matter is treated in a relativistic mean-field approximation and quark matter is modeled by three-flavor local and non-local Nambu Jona-Lasinio (NJL) models with repulsive vector interactions. The NJL models typically yield equations of state that feature a first order transition to quark matter. Assuming that the quark-hadron surface tension is high enough to disfavour mixed phases, and restricting to EoSes that allow stars to reach 2 solar masses, we find that the appearance of the quark matter core either destabilizes the star immediately (this is typical for non-local NJL models) or leads to a very short hybrid star branch in the mass-radius relation (this is typical for local NJL models). Using the Constant-Sound-Speed parametrization we can see that the reason for the near-absence of hybrid stars is that the transition pressure is fairly high and the transition is strongly first order.