Generic conditions for stable hybrid stars

TL;DR

This paper investigates the mass-radius relationship of hybrid stars with a focus on a generic quark matter equation of state, providing a framework to test models and identify conditions for stable, massive hybrid stars.

Contribution

It introduces a generic parameterization of quark matter using the CSS model, enabling comparison and empirical testing of hybrid star models.

Findings

The phase diagram of hybrid star mass-radius relations is sensitive to quark matter parameters.

Hybrid stars with masses above 2 solar masses are possible with reasonable quark matter parameters.

The mass-radius relation is relatively insensitive to nuclear matter details.

Abstract

We study the mass-radius curve of hybrid stars, assuming a single first-order phase transition between nuclear and quark matter, with a sharp interface between the quark matter core and nuclear matter mantle. We use a generic parameterization of the quark matter equation of state, which has a constant, i.e. density-independent, speed of sound ("CSS"). We argue that this parameterization provides a framework for comparison and empirical testing of models of quark matter. We obtain the phase diagram of possible forms of the hybrid star mass-radius relation, where the control parameters are the transition pressure, energy density discontinuity, and the quark matter speed of sound. We find that this diagram is sensitive to the quark matter parameters but fairly insensitive to details of the nuclear matter equation of state. We calculate the maximum hybrid star mass as a function of the parameters of the quark matter EoS, and find that there are reasonable values of those parameters that give rise to hybrid stars with mass above $2\,M_\odot$.