Non-Identical Neutron Star Twins

TL;DR

This paper proposes the theoretical possibility of a third family of stable, high-density degenerate stars caused by first order phase transitions, challenging previous stability limits for neutron stars.

Contribution

It introduces a class of equations of state with phase transitions that allow for stable, higher-density degenerate stars, supported by principles like causality and general relativity.

Findings

Existence of a third family of stable degenerate stars.

Potential observational methods via X-ray oscillations.

Theoretical proof based on fundamental physical principles.

Abstract

The work of J. A. Wheeler in the mid 1960's showed that for smooth equations of state no stable stellar configurations with central densities above that corresponding to the limiting mass of ``neutron stars'' (in the generic sense) were stable against acoustical vibrational modes. A perturbation would cause any such star to collapse to a black hole or explode. Accordingly, there has been no reason to expect that a stable degenerate family of stars with higher density than the known white dwarfs and neutron stars might exist. We have found a class of exceptions corresponding to certain equations of state that describe a first order phase transition. We discuss how such a higher density family of stars could be formed in nature, and how the promising new exploration of oscillations in the X-ray brightness of accreting neutron stars might provide a means of identifying them. Our proof of the possible existence of a third family of degenerate stars is one of principle and rests on general principles like causality, microstability of matter and General Relativity.