Dark matter effects on the properties of hybrid neutron stars

TL;DR

The paper addresses how dark matter (DM) embedded in hybrid neutron stars (DHSs) affects their global properties and the hadron–quark phase transition. It adopts a gravity-only two-fluid framework, employing Brueckner–Hartree–Fock for nuclear matter, Dyson–Schwinger or Field Correlator Model for quark matter, and a non-self-annihilating self-interacting fermionic DM EOS with $\mu=1\ \text{GeV}$, constructing DHS configurations via Gibbs phase transition. The main findings show that DM lowers the onset mass for quark matter, can create DM-core or DM-halo stellar structures, and reduces radial oscillation frequencies, with the effects sensitive to the DM fraction and the quark EOS (notably with the V18+DS1 model). These results imply DM can significantly influence the interpretation of NS observations, including mass–radius measurements and pulsar timing signals, and highlight potential observable signatures of DM in compact objects.

Abstract

We study the effects of dark matter on the properties of hybrid neutron stars, in particular the influence on the mass-radius relation, the value of the maximum mass, and the hadron-quark phase transition. To single out the equilibrium configurations of dark-matter-admixed hybrid neutron stars (DHSs), we also study their radial oscillations. Both the stellar structure equations and the radial oscillation equations are solved for the two-fluid system, where the ordinary matter component and dark matter component couple only through gravity. For the ordinary matter components, we adopt the Brueckner-Hartree-Fock method for nuclear matter, and the Dyson-Schwinger or the field-correlator model for quark matter. For the dark matter component, we use a non-self-annihilating self-interacting fermionic model. We find that the presence of dark matter in DHSs leads to a decrease of the critical mass of the hadron-quark phase transition, a related possible onset of quark matter in dark-matter accreting stars, and a significant reduction of radial oscillation frequencies.

Figures (8)

• Figure 1: (a) Stellar EOSs of nuclear matter (solid black curve), hybrid matter with Gibbs phase transition construction (broken colored), and for DM (solid lilac); (b) Squared speed of sound as function of energy density $\epsilon$ for the selected EOSs. Markers indicate the onset of QM and the $M_\text{max}$ configurations of the corresponding stars.
• Figure 2: Mass-radius relation for the various EOSs. Observational constraints on masses Fonseca21Romani22 and radii $R_{1.4}$, $R_{2.0}$, and $R_{2.08}$ from NICER Miller21Rutherford24 (horizontal bars) are included.
• Figure 3: DHS mass $(M_G,M_B)$ vs radius $R=\max(R_N,R_D)$ relations for different DM fractions $f=M_D/M=0,0.1,0.2,...1$, for various EOSs. The solid black curves indicate the sequence of maximum masses with varying $f$. The broken red curves indicate the critical configurations of the hadron-quark phase transition onset. Dotted black horizontal lines indicate $M_\text{max}$ of pure nucleonic and dark stars. Markers and dashed red lines indicate the range of possible HSs. See text for further details.
• Figure 4: The stable DHS configurations in the $(f,M_G)$ plane for the different EOSs. The $M_\text{max}$ and $M_\text{pt}$ curves correspond to those in Fig. \ref{['f:mrf']}. The DM-core and DM-halo domains are emphasized, separated by the $R_N=R_D$ dotted green curves.
• Figure 5: The allowed domain of DHSs with the EOS V18+DS1 in the $(f,M_B)$ plane. Notation is as in Fig. \ref{['f:mf']}. Contours of gravitational mass $M_G$ are shown by dashed lines. Horizontal green lines indicate fixed-$M_B$ configurations with initial $M_G(f=0)=1.0,1.2,1.4,1.6\,M_\odot$.