arXiv subjects

Alessandro Parisi

Publications and source records attributed to Alessandro Parisi.

6 recordsLinked to original sources

arXiv2026

$f$-Mode oscillations and the gravitational response of compact stars with analytic equations of state

We use a simple holographic model to study the property of cold and dense neutron stars (NSs) and deconfined QCD matter. With the aim of investigating the global properties of compact stars, such as the total gravitational mass and radius, the equation of states (EOS) of neutron stars and quark stars (QSs) are used to solve the Tolman-Oppenheimer-Volkov (TOV) equations for stellar structure. Additionally, we investigate the tidal deformabilities and $f$-mode oscillation for these two different compact stars. Our main conclusion is that, by using a holographic equation of state, it is possible to obtain neutron matter and quark matter properties and that it is also possible to extend the procedure to astrophysical applications.

hep-ph↗

arXiv2022

Self-interacting Dark Scalar Spikes around Black Holes via Relativistic Bondi Accretion

We consider the spike mass density profile in a dark halo by self-consistently solving the relativistic Bondi accretion of dark matter onto a non-spining black hole of mass $M$. We assume that the dominant component of the dark matter in the halo is a Standard model gauge-singlet scalar. Its mass $m\simeq 10^{-5}{\rm eV}$ and quartic self-coupling $λ\lesssim10^{-19}$ are constrained to be compatible with the properties of galactic dark halos. In the hydrodynamic limit, we find that the accretion rate is bounded from below, $\dot{M}_{\rm min}=96πG^2M^2 m^4/λ\hbar^3$. Therefore, for $M=10^6~{\rm M}_\odot$ we have $\dot{M}_{\rm min}\simeq1.41\times 10^{-9}~{\rm M}_\odot~{\rm yr}^{-1}$, which is subdominant compared to the Eddington accretion of baryons. The spike density profile $ρ_0(r)$ within the self-gravitating regime cannot be fitted well by a single-power law but a double-power one. Despite that, we can fit $ρ_0(r)$ piecewise and find that $ρ_0(r) \propto r^{-1.20}$ near the sound horizon, $ρ_0(r) \propto r^{-1.00}$ towards the Bondi radius and $ρ_0(r) \propto r^{-1.08}$ for the region in between. This contrasts with more cuspy $ρ_0(r) \propto r^{-1.75}$ for dark matter with Coulomb-like self-interaction.

astro-ph.HE↗

arXiv2020

Hybrid stars in the light of the merging event GW170817

We study quark-hadron hybrid stars with sharp phase transitions assuming that phase conversions at the interface are slow. Hadronic matter is described by a set of equations of state (EoS) based on the chiral effective field theory and quark matter by a generic bag model. Due to slow conversions at the interface, there is an extended region of stable hybrid stars with central densities above the density of the maximum mass star. We explore systematically the role of the transition pressure and the energy-density jump $Δε$ at the interface on some global properties of hybrid stars. We find that for a given transition pressure, the radius of the last stable hybrid star decreases as $Δε$ raises resulting in a larger extended branch of stable hybrid stars. Contrary to purely hadronic stars, the tidal deformability $Λ$ can be either a decreasing or an increasing function of the stellar mass $M$ and for large values of the transition pressure has a very weak dependence on $M$. Finally, we analyze the tidal deformabilities $Λ_1$ and $Λ_2$ for a binary system with the same chirp mass as GW170817. In the scenario where at least one of the stars in the binary is hybrid, we find that models with low enough transition pressure are inside the $90 \%$ credible region of GW170817. However, these models have maximum masses below $2 \, M_{\odot}$, in disagreement with observations. We also find that the LIGO/Virgo constrain (at $90\%$ level) and the $2 \, M_{\odot}$ requirement can be simultaneously fulfilled in a scenario where all hybrid configurations have masses larger than $1.6 \, M_{\odot}$ and the hadronic EoS is not too stiff, such as several of our hybrid models involving a hadronic EoS of intermediate stiffness. In such scenario hybrid stars may exist in Nature but both objects in GW170817 were hadronic stars.

astro-ph.HE↗

arXiv2019

Fundamental oscillation modes of scalar bosonic dark stars

We perform a detailed analysis of the fundamental $f$-mode frequencies and damping times of nonrotating boson stars in general relativity by solving the nonradial perturbation equations. Two parameters which govern the microscopic properties of the bosonic condensates, namely the self-coupling strength and the mass of scalar particle, are explored. These two quantities characterize oscillations of boson star. Specifically, we reexamine some empirical relations that describe the $f$-mode parameters in terms of mass and radius of the boson stars. We found it is possible to constrain the equation of state if the fundamental oscillation mode is observed.

hep-ph↗

arXiv2015

Torsional oscillations of nonbare strange stars

Strange stars are one of the possible compact stellar objects that can be formed after a supernova collapse. We consider a model of strange star having an inner core in the color-flavor locked phase surmounted by a crystalline color superconducting layer. These two phases constitute the {\it quarksphere}, which we assume to be the largest and heaviest part of the strange star. The next layer consists of standard nuclear matter forming a ionic crust, hovering on the top of the quarksphere and prevented from falling by a strong dipolar electric field. The dipolar electric field arises because quark matter is confined in the quarksphere by the strong interaction, but electrons can leak outside forming a few hundreds Fermi thick electron layer separating the ionic crust from the underlying quark matter. The ionic matter and the crystalline color superconducting matter constitute two electromagnetically coupled crust layers. We study the torsional oscillations of these two layers. Remarkably, we find that if a fraction larger than $10^{-4}$ of the energy of a Vela-like glitch is conveyed to a torsional oscillation, the ionic crust will likely break. The reason is that the very rigid and heavy crystalline color superconducting crust layer will absorb only a small fraction of the glitch energy, leading to a large amplitude torsional oscillation of the ionic crust.

astro-ph.SR↗

arXiv2014

Electromagnetic signals from bare strange stars

The crystalline color superconducting phase is believed to be the ground state of deconfined quark matter for sufficiently large values of the strange quark mass. This phase has the remarkable property of being more rigid than any known material. It can therefore sustain large shear stresses, supporting torsional oscillations of large amplitude. The torsional oscillations could lead to observable electromagnetic signals if strange stars have a crystalline color superconducting crust. Indeed, considering a simple model of strange star with a bare quark matter surface, it turns out that a positive charge is localized in a narrow shell about ten Fermi thick beneath the star surface. The electrons needed to neutralize the positive charge of quarks spill in the star exterior forming an electromagnetically bounded atmosphere hundreds of Fermi thick. When a torsional oscillation is excited, for example by a stellar glitch, the positive charge oscillates with typical kHz frequencies, for a crust thickness of about one-tenth of the stellar radius, to hundreds of Hz, for a crust thickness of about nine-tenths of the stellar radius. Higher frequencies, of the order of few GHz, can be reached if the star crust is of the order of few centimeters thick. We estimate the emitted power considering emission by an oscillating magnetic dipole, finding that it can be quite large, of the order of $10^{45}$ erg/s for a thin crust. The associated relaxation times are very uncertain, with values ranging between microseconds and minutes, depending on the crust thickness. The radiated photons will be in part absorbed by the electronic atmosphere, but a sizable fraction of them should be emitted by the star.

hep-ph↗