Hybrid strangeon stars

Chen Zhang, Yong Gao, Cheng-Jun Xia, and Renxin Xu

Phys. Rev. D 108, 123031 – Published 19 December 2023

Abstract

It was conjectured that the basic units of the ground state of bulk strong matter may be strange-clusters called strangeons, and they can form self-bound strangeon stars that are highly compact. Strangeon stars can develop a strange quark matter (SQM) core at high densities, particularly in the color-flavor-locking phase, yielding a branch of hybrid strangeon stars. We explore the stellar structure and astrophysical implications of hybrid strangeon stars. We find that hybrid strangeon stars can meet various astrophysical constraints on pulsar masses, radii, and tidal deformabilities. Finally, we show that the strangeon-SQM mixed phase is not preferred if the charge-neutrality condition is imposed at the strangeon-SQM transition region.

Received 4 October 2023
Accepted 21 November 2023

DOI:https://doi.org/10.1103/PhysRevD.108.123031

Physics Subject Headings (PhySH)

Exotic phases of matter Gravitational wave sources QCD phase transitions Quark matter

Neutron stars & pulsars Stars Stellar interior

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Chen Zhang ^1,*, Yong Gao ^2,3,4,†, Cheng-Jun Xia ^5,‡, and Renxin Xu ^3,4,§

¹The HKUST Jockey Club Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong SAR, People’s Republic of China
²Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany
³Department of Astronomy, School of Physics, Peking University, Beijing 100871, China
⁴Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
⁵Center for Gravitation and Cosmology, College of Physical Science and Technology, Yangzhou University, Yangzhou 225009, China

^*iasczhang@ust.hk
^†gaoyong.physics@pku.edu.cn
^‡cjxia@yzu.edu.cn
^§r.x.xu@pku.edu.cn

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Vol. 108, Iss. 12 — 15 December 2023

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Images

Figure 1
Allowed parameter space (blue-shaded) for the existence of hybrid strangeon stars from stability consideration $(E / A)_{Strangeon} < (E / A)_{CFL}$ . Top: CFL bag constant $B$ and bottom: CFL superconductivity gap $Δ$ versus parameter $ε / N_{q}$ of strangeon matter. For the bottom sub-figure, the shaded region with lighter-colored contour lines represents larger bag constant, sampling $B = 60, 80, 100, 120 MeV / {fm}^{3}$ (bottom to top).
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Figure 2
The curves $M − R$ (left) and $Λ − M$ (right) of hybrid strangeon stars (solid lines) with $ε / N_{q} = 80 / 9 \approx 8.9$ (black), $120 / 9 \approx 13.3$ (blue) MeV, $n_{s} = 0.22$ (thin), 0.30 (thick) $f m^{- 3}$ for the strangeon composition, and $B = 60$ (top), 80 (bottom) $MeV / {fm}^{3}$ for the CFL composition. Lines with darker colors denote larger $Δ$ , sampling 60, 80 MeV for top panels and 60, 100, 120 MeV for bottom panels, respectively. (no large- $Δ$ lines in top panels due to stability constraints referring to Fig. 1.) Dashed lines are pure strangeon star configurations. Shaded regions are constraints with 90% credibility from the NICER mission PSR $J 0030 + 0451$ (green colored) [83, 84], PSR $J 0740 + 6620$ (cyan colored) [85, 86]. The cyan-dotted vertical line in the right panels denotes the GW170817’s $Λ (1.4 M_{⊙}) \leq 800$ constraint [4].
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Figure 3
The relations of $ρ (P)$ for stable hybrid strangeon stars, with $ε / N_{q} = 80 / 9 MeV$ , $n_{s} = 0.22$ (thin), 0.30 (thick) $f m^{- 3}$ for the strangeon composition, and $B = 60$ (green dashed), 80 (black solid) $MeV / {fm}^{3}$ for the CFL composition. Lines with darker colors denote larger $Δ$ , sampling 60, 80 MeV for green lines and 100, 120 MeV for black lines, respectively.
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Figure 4
Mass (left) and radius (right) versus center pressure, $P_{c}$ , for hybrid strangeon stars with strangeon crusts of ${n_{s} = 0.22 / {fm}^{3}, \tilde{ε} = 80 / 9 MeV}$ , and CFL cores of ${B = 60 MeV / {fm}^{3}, Δ = 60, 80 MeV}$ (top) and ${B = 80 MeV / {fm}^{3}, Δ = 100, 120 MeV}$ (bottom). Darker color denotes larger $Δ$ values. Dashed lines denote pure strangeon stars. Solid lines denote hybrid strangeon stars. The dot-dashed lines denote the CFL cores. The right ends of the solid and dot-dashed lines are truncated at the corresponding maximum mass points.
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Figure 5
Pressure is plotted as a function of $μ_{B}$ and $μ_{e}$ for strangeon phase (green) and strange quark matter (red) of normal unpaired (top panel) and charged CFL phase (bottom panel) of $Δ = 100 MeV$ . The mixed phase sits in the intersection of the two surfaces. For illustration, here $B = 80 MeV / {fm}^{3}$ , $m_{s} = 95 MeV$ for the SQM phase and $ε / N_{q} = 80 / 9, n_{s} = 0.3 / {fm}^{3}$ for strangeon phase.
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