Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Revisiting the Possibility of a Sharp Phase Transition in Cold Neutron Stars

Bao-Jun Cai, Bao-An Li, Yu-Gang Ma

Abstract

First-order phase transitions (FOPTs) in cold neutron stars (NSs) have been extensively studied and have provided valuable insights into the behavior of the densest matter visible in our Universe, although a strong consensus has yet to emerge. Revisiting the possibility of a hadron-quark FOPT from a new perspective, we examine the interplay between the coupled nature of gravity and microscopic interactions in Tolman--Oppenheimer--Volkoff (TOV) equations and the fundamental requirements of thermodynamic consistency in NSs. We demonstrate that a sharp FOPT manifested as a plateau in the equation of state (EOS) $P(\varepsilon)$, i.e., pressure $P$ versus energy density $\varepsilon$, is intrinsically incompatible with the regularity conditions of the TOV solutions. Although numerical integrations of the TOV equations with EOSs incorporating FOPTs may yield seemingly reasonable mass-radius relations consistent with current observations, such results can mask underlying inconsistencies. Our analysis thus establishes a structural consistency criterion for constraining dense-matter EOSs using NS observables, complementing existing studies of possible phase transitions in NS interiors.

Revisiting the Possibility of a Sharp Phase Transition in Cold Neutron Stars

Abstract

First-order phase transitions (FOPTs) in cold neutron stars (NSs) have been extensively studied and have provided valuable insights into the behavior of the densest matter visible in our Universe, although a strong consensus has yet to emerge. Revisiting the possibility of a hadron-quark FOPT from a new perspective, we examine the interplay between the coupled nature of gravity and microscopic interactions in Tolman--Oppenheimer--Volkoff (TOV) equations and the fundamental requirements of thermodynamic consistency in NSs. We demonstrate that a sharp FOPT manifested as a plateau in the equation of state (EOS) , i.e., pressure versus energy density , is intrinsically incompatible with the regularity conditions of the TOV solutions. Although numerical integrations of the TOV equations with EOSs incorporating FOPTs may yield seemingly reasonable mass-radius relations consistent with current observations, such results can mask underlying inconsistencies. Our analysis thus establishes a structural consistency criterion for constraining dense-matter EOSs using NS observables, complementing existing studies of possible phase transitions in NS interiors.

Paper Structure

This paper contains 5 sections, 18 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Physical Requirements Forbidding a FOPT in NSs
  3. TOV Integrations Cannot Detect Unphysical EOSs
  4. Model Illustrations
  5. Summary and Outlook

Figures (2)

  • Figure 1: (Color Online). Sketch of a FOPT: a plateau in the EOS correspondingly produces a plateau in the pressure as a function of radial coordinate.
  • Figure 2: (Color Online). First row: NS M-R curves with a FOPT (panel (b)), where $\Delta\varepsilon = \varepsilon_{\rm b} - \varepsilon_{\rm a}$ is randomly selected between 0 and $\varepsilon_0$. Panel (a) shows an example EOS, and the magenta line in panel (b) corresponds to the EOS shown in panel (a). Second row (panels (c) and (d)): Same as the first row, but allowing a negative effective SSS parameter in the range of $-0.1 \lesssim \beta \lesssim 0$.