arXiv subjects

David Edwin Alvarez-Castillo

Publications and source records attributed to David Edwin Alvarez-Castillo.

6 recordsLinked to original sources

arXiv2024

Probing hadron-quark phase transition in twin stars using $f$-modes

Although it is conjectured that a phase transition from hadronic to deconfined quark matter in the ultrahigh-density environment of Neutron Stars (NS), the nature of phase transition remains an unresolved mystery. Furthermore, recent efforts reveal that the finite surface tension effects can lead to a mixed phase with different geometric shapes (so-called "pasta" phases), leading to a smooth phase transition from hadronic to quark matter in the NS interior. Depending on whether there is a strong or a pasta-induced smooth first-order phase transition, one may expect a third family of stable, compact stars or "twin stars" to appear, with the same mass but different radii compared to NSs. The possibility of identifying twin stars using astrophysical observations has been a subject of interest. This study investigates the potential of probing the nature of the hadron-quark phase transition through future gravitational wave (GW) detections from fundamental ($f$-) mode oscillations in Neutron Stars. Using a newly developed model that parametrizes the hadron-quark phase transition with ``pasta phases," we calculate $f$-mode characteristics within a full general relativistic framework. We then use Universal Relations in GW asteroseismology to derive stellar properties from the detected mode parameters. Our findings suggest that detecting GWs from $f$-modes with third-generation GW detectors offers a promising scenario for the existence of twin stars. However, we also estimate various uncertainties in determining the mode parameters and conclude that these uncertainties make it more challenging to identify the nature of the hadron-quark phase transition.

nucl-th↗

arXiv2019

Accretion-induced collapse to third family compact stars as trigger for eccentric orbits of millisecond pulsars in binaries

A numerical rotating neutron star solver is used to study the temporal evolution of accreting neutron stars using a multi-polytrope model for the nuclear equation of state named ACB5. The solver is based on a quadrupole expansion of the metric, but confirms the results of previous works, revealing the possibility of an abrupt transition of a neutron star from a purely hadronic branch to a third-family branch of stable hybrid stars, passing through an unstable intermediate branch. The accretion is described through a sequence of stationary rotating {stellar} configurations which lose angular momentum through magnetic dipole emission while, at the same time, gaining angular momentum through mass accretion. The model has several free parameters which are inferred from observations. The mass accretion scenario is studied in dependence on the effectiveness of angular momentum transfer which determines at which spin frequency the neutron star will become unstable against gravitational collapse to the corresponding hybrid star on the stable third-family branch. It is conceivable that the neutrino burst which accompanies the deconfinement transition may trigger a pulsar kick which results in the eccentric orbit. A consequence of the present model is the prediction of a correlation between the spin frequency of the millisecond pulsar in the eccentric orbit and its mass at birth.

astro-ph.HE↗

arXiv2019

Astrophysical aspects of general relativistic mass twin stars

In this chapter we will introduce an effective equation of state (EoS) model based on polytropes that serves to study the so called "mass twins" scenario, where two compact stars have approximately the same mass but (significant for observation) quite different radii. Stellar mass twin configurations are obtained if a strong first-order phase transition occurs in the interior of a compact star. In the mass-radius diagram of compact stars, this will lead to a third branch of gravitationally stable stars with features that are very distinctive from those of white dwarfs and neutron stars. We discuss rotating hybrid star sequences in the slow rotation approximation and in full general relativity and draw conclusions for an upper limit on the maximum mass of nonrotating compact stars that has recently be deduced from the observation of the merger event GW170817.

astro-ph.HE↗

arXiv2012

Symmetry energy effects in the neutron star properties

The functional form of the nuclear symmetry energy has only been determined in a very narrow range of densities. Uncertainties concern both the low as well as the high density behaviour of this function. In this work different shapes of the symmetry energy, consistent with the experimental data, were introduced and their consequences for the crustal properties of neutron stars are presented. The resulting models are in agreement with astrophysical observations.

astro-ph.SR↗

arXiv2012

Systematic analysis of symmetry energy effects in the neutron star crust properties

The functional form of the nuclear symmetry energy in the whole range of densities relevant for the neutron stars is still unknown. Discrepancies concern both the low as well as the high density behaviour of this function. By use of Bezier curves three different families of the symmetry energy shapes, relevant for different density range were introduced. Their consequences for the crustal properties of neutron stars are presented.

astro-ph.SR↗

arXiv2011

Breaking Pseudo-Rotational Symmetry through ${\bf H}^2_+$ Metric Deformation in the Eckart Potential Problem

The peculiarity of the Eckart potential problem on ${\bf H}^2_+$ (the upper sheet of the two-sheeted two-dimensional hyperboloid), to preserve the $(2l+1)$-fold degeneracy of the states typical for the geodesic motion there, is usually explained in casting the respective Hamiltonian in terms of the Casimir invariant of an so(2,1) algebra, referred to as potential algebra. In general, there are many possible similarity transformations of the symmetry algebras of the free motions on curved surfaces towards potential algebras, which are not all necessarily unitary. In the literature, a transformation of the symmetry algebra of the geodesic motion on ${\bf H}^2_+$ towards the potential algebra of Eckart's Hamiltonian has been constructed for the prime purpose to prove that the Eckart interaction belongs to the class of Natanzon potentials. We here take a different path and search for a transformation which connects the $(2l+1)$ dimensional representation space of the pseudo-rotational so(2,1) algebra, spanned by the rank-$l$ pseudo-spherical harmonics, to the representation space of equal dimension of the potential algebra and find a transformation of the scaling type. Our case is that in so doing one is producing a deformed isometry copy to ${\bf H}^2_+$ such that the free motion on the copy is equivalent to a motion on ${\bf H}^2_+$, perturbed by a $\coth$ interaction. In this way, we link the so(2,1) potential algebra concept of the Eckart Hamiltonian to a subtle type of pseudo-rotational symmetry breaking through ${\bf H}^2_+$ metric deformation.

math-ph↗