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A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Compact Objects".

Deadline for manuscript submissions: closed (30 November 2021) | Viewed by 12720

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Special Issue Editors

Prof. Dr. Júlio César Fabris

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Núcleo Cosmo UFES, Universidade Federal do Espírito Santo (UFES), Campus Goiabeiras, 14 - Goiabeiras, Vitória 29075-910, ES, Brazil
Interests: cosmology; dark energy; black holes; quantum cosmology
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Dr. Raissa F. P. Mendes

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Instituto de Física, Universidade Federal Fluminense, Niterói, Rio de Janeiro 24210-346, Brazil

Dr. Nelson Pinto Neto

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Department of Physiology, Universidade Federal de São Paulo UNIFESP, São Paulo, SP, Brazil

Prof. Dr. Hermano Velten

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Departamento de Física, Universidade Federal de Ouro Preto (UFOP), 35.400-000 Ouro Preto, Minas Gerais, Brazil
Interests: cosmology; astrophysics; dark matter; dark energy; general relativity and its extensions
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Dr. Gustavo Dotti

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Facultad de Matemática, Astronomía y Física (FaMAF), Universidad Nacional de Córdoba and Instituto de Física Enrique Gaviola, CONICET. Ciudad Universitaria, (5000) Córdoba, Argentina

Special Issue Information

Dear Colleagues,

The concept of compact astrophysical objects includes stars and remnants of stars, e.g., black holes, white dwarfs, neutron stars, and also some more hypothetical astrophysical objects like boson stars. Compact objects are of great importance, first, due to their very peculiar features and properties and, second, because they can be very powerful tools to test astrophysical models for stars as well as modified gravity theories. At the same time, these objects allow us to study matter under extreme conditions, involving concepts not only of gravity theory, but also nuclear physics, particle physics, and other related branches of physics.

The 5th José Plínio Baptista School of Cosmology, planned to take place in Guarapari, ES, Brazil, from September 30 - October 5, 2021, will be entirely devoted to Compact Astrophysical Objects. This Special Issue of Universe will include contributions from the speakers of the JBPCosmo School as well as other invited authors. Any contribution from researchers working on this theme will be considered. The goal is to give a general view, as complete as possible, of the state of art of this dynamic subject.

Prof. Dr. Júlio César Fabris
Dr. Raissa F. P. Mendes
Dr. Nelson Pinto Neto
Prof. Dr. Hermano Velten
Dr. Gustavo Dotti
Guest Editors

Manuscript Submission Information

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Published Papers (5 papers)

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Research

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14 pages, 1284 KiB

Open AccessArticle

Neutron Stars in the Symmetron Model

by Bernardo F. de Aguiar, Raissa F. P. Mendes and Felipe T. Falciano

Universe 2022, 8(1), 6; https://doi.org/10.3390/universe8010006 - 23 Dec 2021

Cited by 8 | Viewed by 2475

Abstract

Screening mechanisms are often deployed by dark energy models to conceal the effects of their new degrees of freedom from the scrutiny of terrestrial and solar system experiments. However, the extreme properties of nuclear matter may lead to a partial failure of screening mechanisms inside the most massive neutron stars observed in nature, opening up the possibility of probing these theories with neutron star observations. In this work, we explore equilibrium and stability properties of neutron stars in two variants of the symmetron model. We show that around sufficiently compact neutron stars, the symmetron is amplified with respect to its background (cosmological) value by several orders of magnitude, and that the properties of such unscreened stars are sensitive to corrections to the leading linear coupling between the symmetron and matter. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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32 pages, 4722 KiB

Open AccessArticle

A Magnetospheric Dichotomy for Pulsars with Extreme Inclinations

by Fan Zhang

Universe 2021, 7(12), 455; https://doi.org/10.3390/universe7120455 - 23 Nov 2021

Viewed by 1852

Abstract

In this work, we expand on a comment by Lyne et al. (2017), that intermittent pulsars tend to congregate near a stripe in the logarithmic period versus period-derivative diagram. Such a stripe represents a small range of polar cap electric potential. Taking into account also the fact (already apparent in their Figure 7, but not explicitly stated there) that high-fraction nulling pulsars also tend to reside within this and an additional stripe, we make the observation that the two stripes further match the “death lines” for double- and single-pole interpulses, associated with nearly orthogonal and aligned rotators, respectively. These extreme inclinations are known to suffer from pair production deficiencies, so we propose to explain intermittency and high-fraction nulling by reinvigorating some older quiescent (no pulsar wind or radio emission) “electrosphere” solutions. Specifically, as the polar potential drops below the two threshold bands (i.e., the two stripes), corresponding to the aligned and orthogonal rotators, their respective magnetospheres transition from being of the active pair-production-sustained-type into becoming the electrospheres, in which charges are only lifted from the star. The borderline cases sitting in the gap outside of the stable regime of either case manifest as high-fraction nullers. Hall evolution of the magnetic field inside orthogonally rotating neutron stars can furthermore drive secular regime changes, resulting in intermittent pulsars. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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15 pages, 2220 KiB

Open AccessArticle

Quasi-Periodic Oscillatory Motion of Particles Orbiting a Distorted, Deformed Compact Object

by Shokoufe Faraji and Audrey Trova

Universe 2021, 7(11), 447; https://doi.org/10.3390/universe7110447 - 19 Nov 2021

Cited by 3 | Viewed by 1458

Abstract

This work explores the dynamic properties of test particles surrounding a distorted, deformed compact object. The astrophysical motivation was to choose such a background as to constitute a more reasonable model of a real situation that arises in the vicinity of compact objects with the possibility of having parameters such as the extra physical degrees of freedom. This can facilitate associating observational data with astrophysical systems. This work’s main goal is to study the dynamic regime of motion and quasi-periodic oscillation in this background, depending on different parameters of the system. In addition, we exercise the resonant phenomena of the radial and vertical oscillations at their observed quasi-periodic oscillations frequency ratio 3:2 and show that the oscillatory frequencies of charged particles can be adequately related to the frequencies of the twin high-frequency quasi-periodic oscillations observed in some sources of the microquasar observational data. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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68 pages, 9866 KiB

Open AccessArticle

The Clustering Dynamics of Primordial Black Holes in N-Body Simulations

by Manuel Trashorras, Juan García-Bellido and Savvas Nesseris

Universe 2021, 7(1), 18; https://doi.org/10.3390/universe7010018 - 15 Jan 2021

Cited by 52 | Viewed by 4278

Abstract

We explore the possibility that Dark Matter (DM) may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian N-body code. We compute the cluster rate of evaporation and extract the binary and merged sub-populations along with their parent and merger tree histories, lifetimes and formation rates, the dynamical and orbital parameter profiles, the degree of mass segregation and dynamical friction and power spectrum of close encounters. Overall, we find that PBHs can constitute a viable DM candidate, and that their clustering presents a rich phenomenology throughout the history of the Universe. We show that binary systems constitute about 9.5% of all PBHs at present, with mass ratios of

{\bar{q}}_{B} = 0.154

, and total masses of

{\bar{m}}_{T, B} = 303 M_{⊙}

. Merged PBHs are rare, about 0.0023% of all PBHs at present, with mass ratios of

{\bar{q}}_{B} = 0.965

with total and chirp masses of

{\bar{m}}_{T, B} = 1670 M_{⊙}

and

{\bar{m}}_{c, M} = 642 M_{⊙}

, respectively. We find that cluster puffing up and evaporation leads to bubbles of these PBHs of order 1 kpc containing at present times about 36% of objects and mass, with one-hundred pc-sized cores. We also find that these PBH sub-haloes are distributed in wider PBH haloes of order hundreds of kpc, containing about 63% of objects and mass, coinciding with the sizes of galactic halos. We find at last high rates of close encounters of massive Black Holes (

M \sim 1000 M_{⊙}

), with

Γ^{S} = (1.2 \begin{matrix} + 5.9 \\ - 0.9 \end{matrix}) \times 10^{7} {yr}^{- 1} {Gpc}^{- 3}

and mergers with

Γ^{M} = 1337 \pm 41 {yr}^{- 1} {Gpc}^{- 3}

. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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Review

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24 pages, 440 KiB

Open AccessReview

Linear Stability of Black Holes and Naked Singularities

by Gustavo Dotti

Universe 2022, 8(1), 38; https://doi.org/10.3390/universe8010038 - 10 Jan 2022

Cited by 5 | Viewed by 1697

Abstract

A review of the current status of the linear stability of black holes and naked singularities is given. The standard modal approach, that takes advantage of the background symmetries and analyze separately the harmonic components of linear perturbations, is briefly introduced and used to prove that the naked singularities in the Kerr–Newman family, as well as the inner black hole regions beyond Cauchy horizons, are unstable and therefore unphysical. The proofs require a treatment of the boundary condition at the timelike boundary, which is given in detail. The nonmodal linear stability concept is then introduced, and used to prove that the domain of outer communications of a Schwarzschild black hole with a non-negative cosmological constant satisfies this stronger stability condition, which rules out transient growths of perturbations, and also to show that the perturbed black hole settles into a slowly rotating Kerr black hole. The encoding of the perturbation fields in gauge invariant curvature scalars and the effects of the perturbation on the geometry of the spacetime is discussed. These notes follow from a course delivered at the V José Plínio Baptista School of Cosmology, held at Guarapari (Espírito Santo) Brazil, from 30 September to 5 October 2021. Full article

(This article belongs to the Special Issue Compact Astrophysical Objects)

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