arXiv subjects

Euaggelos E. Zotos

Publications and source records attributed to Euaggelos E. Zotos.

At least 19 recordsLinked to original sources

arXiv2026

Chiral Vortical Transport under Minimal-Length Deformations in Rotating Relativistic Matter

We study how a minimal length can affect the chiral vortical effect, in which rotation produces a current in matter made of chiral fermions. We focus on an isotropic modification of the momentum-space density-of-state (DoS) and determine the vortical conductivity from the static, long-wavelength correlation between the current and momentum density. The analysis is carried out first for a single right-handed Weyl fermion and is then extended to general massless chiral fermion systems with mutually commuting conserved charges. We find that, for a broad class of analytic isotropic DoS modifications, the familiar leading contributions from chemical potential and temperature remain unchanged at first order within the DoS sector. Minimal-length effects instead appear through higher-order thermal and chemical-potential contributions. Some of these corrections contain the same chiral charge combinations that appear in gauge and mixed gauge-gravitational anomalies, while another contains an independent higher-order charge combination. Their coefficients depend on the form of the DoS modification and are therefore not fixed by the usual four-dimensional anomaly structure. For the Kempf-Mangano-Mann deformation, the exact result shows that a positive minimal-length parameter always reduces the DoS contribution to the vortical conductivity. We also find that a Planck-suppressed correction is extremely small under standard heavy-ion chemical freeze-out conditions. These results show that isotropic minimal-length modifications can change higher-order contributions to chiral vortical transport while leaving the familiar leading anomaly-related response unchanged within the DoS sector considered here.

hep-ph↗

arXiv2026

Properties of strange quark stars in $f(Q)$ gravity: A baryon number density-dependent perspective

We study anisotropic strange quark stars in $f(Q)$ gravity, incorporating a baryon-number-density-dependent bag function $B(n)$ into the MIT bag equation of state, $p=[ρ-4B(n)]/3$. Motivated by a Woods-Saxon parametrization, the density dependence of $B(n)$ provides a phenomenological description of the evolution of quark matter with increasing baryon density beyond the conventional constant-bag prescription. Adopting the Finch-Skea metric ansatz, we obtain exact solutions of the modified field equations for anisotropic matter composed of deconfined up, down and strange quarks. We investigate the thermodynamic stability through the energy per baryon, $E_B=ρ_0/n$, as a function of baryon density and identify the regimes corresponding to stable hadronic matter, metastable hybrid matter and absolutely stable strange quark matter. At sufficiently high densities, $E_B$ falls below the corresponding value for $^{56}$Fe, satisfying the stability criterion for strange quark matter. The resulting stability window, $57.55\leq B(n)\leq 91.55~\mathrm{MeV/fm^3}$, supports stable three-flavour quark matter. Solving the generalized Tolman-Oppenheimer-Volkoff equations, we find a maximum stellar mass of $\sim2.46,M_\odot$, demonstrating that the model can accommodate massive compact stars. The resulting configurations satisfy the relevant causality and energy condition constraints, while their stability is supported by the Herrera cracking criterion and the adiabatic index. These results demonstrate that the combined effects of nonmetricity and a density-dependent bag function can substantially enrich the phenomenology of strange quark stars and provide a viable framework for exploring their internal structure and stability in modified gravity.

gr-qc↗

arXiv2024

Behaviours of rip cosmological models in $f(Q,C)$ gravity

In this study, the Universe's rip cosmology theories have been provided for the $f(Q,C)$ gravity theory, where $Q$ and $C$ stand for the non-metricity scalar and boundary term. We assumed $f(Q,C)=αQ^{n}+βC$ and analyzed the nature of the physical parameters for the Little Rip, Big Rip and Pseudo Rip models. In the LR and PR models, the EoS parameter exhibits phantom characteristics but remains closely aligned with the $Λ$CDM line. After investigating the energy conditions, we recognised that our model violates the strong energy constraint. Avoiding singularity situations has been noted in all of these accelerated models. The characteristics of the jerk and snap parameters have been investigated. Our model provides an effective description of the Universe's evolutionary history and fits well with contemporary cosmic data.

gr-qc↗

arXiv2021

Orbit classification in a disk galaxy model with a pseudo-Newtonian central black hole

We numerically investigate the motion of stars on the meridional plane of an axially symmetric disk galaxy model, containing a central supermassive black hole, represented by the Paczynski-Wiita potential. By using this pseudo-Newtonian potential we can replicate important relativistic properties, such as the existence of the Schwarzschild radius. After classifying extensive samples of initial conditions of trajectories, we manage to distinguish between collisional, ordered, and chaotic motion. Besides, all starting conditions of regular orbits are further classified into families of regular orbits. Our results are presented through modern color-coded basin diagrams on several types of two-dimensional planes. Our analysis reveals that both the mass of the black hole (in direct relation with the Schwarzschild radius) as well as the angular momentum play an important role in the character of orbits of stars. More specifically, the trajectories of low angular momentum stars are highly affected by the mass of the black hole, while high angular momentum stars seem to be unaffected by the central black hole. Comparison with previous related outcomes, using Newtonian potentials for the central region of the galaxy, is also made.

astro-ph.GA↗

arXiv2021

Effect of Multipole Moments in the Weak Field Limit of a Black Hole Plus Halo Potential

In this paper, we consider a Newtonian system whose relativistic counterpart describes a superimposed halo with a black hole. Our aim is to determine how the quadrupole and octupole moments affect the nature of the motion of a test particle, moving in the close vicinity of the black hole. The different types of trajectories for the test particle are mainly classified as bounded, collisional, and escaping, by using modern color-coded basin diagrams. Moreover, an additional analysis is carried out for distinguishing between the different types of bounded motion (regular, sticky, and chaotic). Our results strongly indicate that the multipole moments, along with the total orbital energy, highly affect the final state of the test particle, while at the same time the basin geometry of the phase space tends to be highly dominated by collision and escape orbits.

gr-qc↗

arXiv2021

Numerical investigation on the Hill's type lunar problem with homogeneous potential

We consider the planar Hill's lunar problem with a homogeneous gravitational potential. The investigation of the system is twofold. First, the starting conditions of the trajectories are classified into three classes, that is bounded, escaping, and collisional. Second, we study the no-return property of the Lagrange point $L_2$ and we observe that the escaping trajectories are scattered exponentially. Moreover, it is seen that in the supercritical case, with $α\geq 2$, the basin boundaries are smooth. On the other hand, in the subcritical case, with $α< 2$ the boundaries between the different types of basins exhibit fractal properties.

nlin.CD↗

arXiv2020

Short-term stability of particles in the WD J0914+1914 white dwarf planetary system

Nearly all known white dwarf planetary systems contain detectable rocky debris in the stellar photosphere. A glaring exception is the young and still evolving white dwarf WD J0914+1914, which instead harbours a giant planet and a disc of pure gas. The stability boundaries of this disc and the future prospects for this white dwarf to be polluted with rocks depend upon the mass and orbit of the planet, which are only weakly constrained. Here we combine an ensemble of plausible planet orbits and masses to determine where observers should currently expect to find the outer boundary of the gas disc. We do so by performing a sweep of the entire plausible phase space with short-term numerical integrations. We also demonstrate that particle-star collisional trajectories, which would lead to the (unseen) signature of rocky metal pollution, occupy only a small fraction of the phase space, mostly limited to particle eccentricities above 0.75. Our analysis reveals that a highly inflated planet on a near-circular orbit is the type of planet which is most consistent with the current observations.

astro-ph.EP↗

arXiv2020

The grain size survival threshold in one-planet post-main-sequence exoplanetary systems

The size distribution and orbital architecture of dust, grains, boulders, asteroids, and major planets during the giant branch phases of evolution dictate the preponderance and observability of the eventual debris, which have been found to surround white dwarfs and pollute their atmospheres with metals. Here, we utilize the photogravitational planar restricted three-body problem in one-planet giant branch systems in order to characterize the orbits of grains as the parent star luminosity and mass undergo drastic changes. We perform a detailed dynamical analysis of the character of grain orbits (collisional, escape, or bounded) as a function of location and energy throughout giant branch evolution. We find that for stars with main-sequence masses of $2.0M_{\odot}$, giant branch evolution, combined with the presence of a planet, ubiquitously triggers escape in grains smaller than about 1 mm, while leaving grains larger than about 5 cm bound to the star. This result is applicable for systems with either a terrestrial or giant planet, is largely independent of the location of the planet, and helps establish a radiative size threshold for escape of small particles in giant branch planetary systems.

astro-ph.EP↗

arXiv2020

On the dynamics of a seventh-order generalized Hénon-Heiles potential

This paper deals with the derivation and analysis of a seventh-order generalization of the Hénon-Heiles potential. The new potential has axial and reflection symmetries, and finite escape energy with three channels of escape. Based on SALI indicator and exits basins, the dynamic behavior of the seventh-order system is investigated qualitatively in cases of bounded and unbounded movement. Moreover, a quantitative analysis is carried out through the percentage of chaotic orbits and the basin entropy, respectively. After classifying large sets of initial conditions of orbits for several values of the energy constant in both regimes, we observe that when the energy moves away from the critical value, the chaoticity of the system decreases and the basin structure becomes simpler with sharper and well defined bounds. Our results suggest that when the seventh-order contributions of the potential are taken into account, the system becomes less ergodic in comparison with the classical version of the Hénon-Heiles system.

nlin.CD↗

arXiv2019

Measuring the transition between nonhyperbolic and hyperbolic regimes in open Hamiltonian systems

We show that the presence of KAM islands in nonhyperbolic chaotic scattering has deep implications on the unpredictability of open Hamiltonian systems. When the energy of the system increases the particles escape faster. For this reason the boundary of the exit basins becomes thinner and less fractal. Hence, we could expect a monotonous decrease in the unpredictability as well as in the fractal dimension. However, within the nonhyperbolic regime, fluctuations in the basin entropy have been uncovered. The reason is that when increasing the energy, both the size and geometry of the KAM islands undergo abrupt changes. These fluctuations do not appear within the hyperbolic regime. Hence, the fluctuations in the basin entropy allow us to ascertain the hyperbolic or nonhyperbolic nature of a system. In this manuscript we have used continuous and discrete open Hamiltonian systems in order to show the relevant role of the KAM islands on the unpredictability, and the utility of the basin entropy to analyze this kind of systems.

nlin.CD↗

arXiv2019

Escaping from a degenerate version of the four hill potential

We examine the escape from the four hill potential by using the method of grid classification, when polar coordinates are used for expressing the initial conditions of the orbits. In particular, we investigate how the energy of the orbits influences several aspects of the escape dynamics, such as the escape period and the chosen channels of escape. Color-coded basin diagrams are deployed for presenting the basins of escape using multiple types of planes with two dimensions. We demonstrate that the value of the energy highly influences the escape mechanism of the orbits, as well as the degree of fractality of the dynamical system, which is numerically estimated by computing both the fractal dimension and the entropy of the basin boundaries.

nlin.CD↗

arXiv2019

Orbital and escape dynamics in barred galaxies -- IV. Heteroclinic connections

Continuing the series of papers on a new model for a barred galaxy, we investigate the heteroclinic connections between the two normally hyperbolic invariant manifolds sitting over the two index-1 saddle points of the effective potential. The heteroclinic trajectories and the nearby periodic orbits of similar shape populate the bar region of the galaxy and a neighbourhood of its nucleus. Thereby we see a direct relation between the important structures of the interior region of the galaxy and the projection of the heteroclinic tangle into the position space. As a side result, we obtain a detailed picture of the primary heteroclinic intersection surface in the phase space.

astro-ph.GA↗

arXiv2019

Orbit classification in a pseudo-Newtonian Copenhagen problem with Schwarzschild-like primaries

We examine the orbital dynamics of the planar pseudo-Newtonian Copenhagen problem, in the case of a binary system of Schwarzschild-like primaries, such as super-massive black holes. In particular, we investigate how the Jacobi constant (which is directly connected with the energy of the orbits) influences several aspects of the orbital dynamics, such as the final state of the orbits. We also determine how the relativistic effects (i.e., the Schwarzschild radius) affect the character of the orbits, by comparing our results with the classical Newtonian problem. Basin diagrams are deployed for presenting all the different basin types, using multiple types of planes with two dimensions. We demonstrate that both the Jacobi constant as well as the Schwarzschild radius highly influence the character of the orbits, as well as the degree of fractality of the dynamical system.

gr-qc↗

arXiv2019

On the fractal basins of convergence of the libration points in the axisymmetric five-body problem: the convex configuration

In the present work, the Newton-Raphson basins of convergence, corresponding to the coplanar libration points (which act as numerical attractors), are unveiled in the axisymmetric five-body problem, where convex configuration is considered. In particular, the four primaries are set in axisymmetric central configuration, where the motion is governed only by mutual gravitational attractions. It is observed that the total number libration points are either eleven, thirteen or fifteen for different combination of the angle parameters. Moreover, the stability analysis revealed that the all the libration points are linearly stable for all the studied combination of angle parameters. The multivariate version of the Newton-Raphson iterative scheme is used to reveal the structures of the basins of convergence, associated with the libration points, on various types of two-dimensional configuration planes. In addition, we present how the basins of convergence are related with the corresponding number of required iterations. It is unveiled that in almost every cases, the basins of convergence corresponding to the collinear libration point $L_2$ have infinite extent. Moreover, for some combination of the angle parameters, the collinear libration points $L_{1,2}$ have also infinite extent. In addition, it can be observed that the domains of convergence, associated with the collinear libration point $L_1$, look like exotic bugs with many legs and antennas whereas the domains of convergence, associated with $L_{4,5}$ look like butterfly wings for some combinations of angle parameters. Particularly, our numerical investigation suggests that the evolution of the attracting domains in this dynamical system is very complicated, yet a worthy studying problem.

nlin.CD↗

arXiv2019

Unveiling the basins of convergence in the pseudo-Newtonian planar circular restricted four-body problem

The dynamics of the pseudo-Newtonian restricted four-body problem has been studied in the present paper, where the primaries have equal masses. The parametric variation of the existence as well as the position of the libration points are determined, when the value of the transition parameter $ε\in [0, 1]$. The stability of these libration points has also been discussed. Our study reveals that the Jacobi constant as well as transition parameter $ε$ have substantial effect on the regions of possible motion, where the fourth body is free to move. The multivariate version of Newton-Raphson iterative scheme is introduced for determining the basins of attraction in the configuration $(x,y)$ plane. A systematic numerical investigation is executed to reveal the influence of the transition parameter on the topology of the basins of convergence. In parallel, the required number of iterations is also noted to show its correlations to the corresponding basins of convergence. It is unveiled that the evolution of the attracting regions in the pseudo-Newtonian restricted four-body problem is a highly complicated yet worth studying problem.

nlin.CD↗

arXiv2019

On the Newton-Raphson basins of convergence associated with the libration points in the axisymmetric five-body problem: the concave configuration

The axisymmetric five-body problem with the concave configuration has been studied numerically to reveal the basins of convergence, by exploring the Newton-Raphson iterative scheme, corresponding to the coplanar libration points (which act as attractors). In addition, four primaries are set in axisymmetric central configurations introduced by Érdi and Czirják and the motion is governed by mutual gravitational attraction only. The evolution of the positions of libration points is illustrated, as a function of the value of angle parameters. A systematic and rigorous investigation is performed in an effort to unveil how the angle parameters affect the topology of the basins of convergence. In addition, the relation of the domain of basins of convergence with required number of iterations and the corresponding probability distributions are illustrated.

nlin.CD↗

arXiv2019

On the convergence dynamics of the Sitnikov problem with non-spherical primaries

We investigate, using numerical methods, the convergence dynamics of the Sitnikov problem with non-spherical primaries, by applying the Newton-Raphson (NR) iterative scheme. In particular, we examine how the oblateness parameter $A$ influences several aspects of the method, such as its speed and efficiency. Color-coded diagrams are used for revealing the convergence basins on the plane of complex numbers. Moreover, we compute the degree of fractality of the convergence basins on the complex space, as a relation of the oblateness, by using different computational tools, such the fractal dimension as well as the (boundary) basin entropy.

nlin.CD↗

arXiv2019

On the classification of orbits in the three-dimensional Copenhagen problem with oblate primaries

The character of motion for the three-dimensional circular restricted three-body problem with oblate primaries is investigated. The orbits of the test particle are classified into four types: non-escaping regular orbits around the primaries, trapped chaotic (or sticky) orbits, escaping orbits that pass over the Lagrange saddle points $L_2$ and $L_3$, and orbits that lead the test particle to collide with one of the primary bodies. We numerically explore the motion of the test particle by presenting color-coded diagrams, where the initial conditions are mapped to the orbit type and studied as a function of the total orbital energy, the initial value of the $z$-coordinate and the oblateness coefficient. The fraction of the collision orbits, measured on the color-coded diagrams, show an algebraic dependence on the oblateness coefficient, which can be derived by simple semi-theoretical arguments.

nlin.CD↗