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Description
Although the class of stellar remnants that are neither white dwarves nor black holes is traditionally named neutron stars (NS), these objects are not composed solely of neutrons. Even the more naïve description of such objects must include protons and leptons to guarantee their stability. A neutron star was first detected as a pulsar in 1967, and since 2017, when the first gravitational wave resulting from an NS merger was detected, new constraints on the nuclear equations of state (EOS) used to describe NS have been obtained [1]. Also, the first data from the NICER telescope was available in 2021 with more contributions to clarify the scenario [2] after a very massive NS was confirmed. More recently, other exotic objects have been detected, with low masses an radii, HESS J1731-347 [3] and XTE J1814-338 [4].
With the advent of these detections, two conjectures have been explored: 1) the possibility that neutron stars are hybrid objects with a hadronic outer core and a central quark core, an idea proposed in the 60s by Ivanenko and Kurdgelaidze [5] and 2) the effects of dark matter on NS properties [6].
Different prescriptions to build an EOS that can describe hybrid stars exist [7] and are all based on the fact that a phase transition from a hadronic to a deconfined quark phase is possible [8]. Depending on the models used, it is possible to describe a 2.15 solar mass star and also a 2.03 solar mass star containing a quark core that corresponds to more than 80% of its total mass and radius [9], as suggested by the analysis of sound velocity in different stellar media [10]. Both massive hybrid stars [5] and the tiny objects HESS J1731-347 and XTE J1814-338 [11] can also be described if dark matter is considered.
[1] B.P. Abbott et al., Phys. Rev. Lett. 2017, 119, 161101.
[2] M.C. Miller et al, ApJL 918, L28 (2021); T.E. Ridley et al, ApJL 918, L27 (2021).
[3] V. Doroshenko, V. Suleimanov, G. Puhlhofer and A. Santangelo, Nature Astr. 6, 1444 (2022).
[4] Y. Kini, et al., Monthly Notices of the Royal Astronomical Society 527, 8118–8130 (2023).
[5] D.Ivanenko and D.F. Kurdgelaidze, Astrophysics 1965, 1, 251–252.
[6] C.H. Lenzi, M. Dutra, O. Lourenço, L.L. Lopes and D.P. Menezes, Eur. Phys. J. C (2023) 83:266.
[7] D.P. Menezes, Universe, 2021, 7, 267; A. Issifu, F.M. da Silva and D.P. Menezes
Monthly Notices of the Royal Astronomical Society 525 (2023) 5512-5519.
[8] L.L. Lopes, C. Biesdorf and D.P. Menezes, Monthly Notices of the Royal Astronomical Society 512, 5110 (2022).
[9] L.L. Lopes and D.P Menezes, Astroph. J. 936:41 (2022).
[10] E. Annala et al, Nature Phys. 2020, 16, 907–910.
[11] L.L. Lopes and A. Issifu, arXiv 2411.17105.
