arXiv subjects

Prashanth Jaikumar

Publications and source records attributed to Prashanth Jaikumar.

At least 19 recordsLinked to original sources

arXiv2026

Observables and conformal properties of dark matter admixed isentropic neutron stars

We construct an equation of state for isentropic dark-matter-admixed neutron stars (DMANS) with a hot core and relatively cold crust incorporating self-consistent temperature and DM density profiles for GeV-scale fermionic DM. We show that the enhancement of central stellar density due to DM accumulation, previously reported for cold neutron stars, remains robust. Substantial observable effects of DM accumulation arise only for sufficiently massive stellar configurations. Similar to earlier studies of cold NS, the speed of sound profile is shown to exhibit non-monotonic behavior for sufficiently large DM density in the core. We identify a competition between thermal effects due to nonzero values of entropy per baryon and softening effects of the dark sector which drive macroscopic properties and conformality indicators in opposite directions. This competition determines the onset of conformality near the stellar core and indicates that conformality signatures attributed to quark-matter in cold NS could be mimicked by DM admixture in isentropic stars.

hep-ph↗

arXiv2026

Finite temperature effects on g-modes of inviscid neutron stars

We study the effect of temperature on secular, compositional $g$-modes in the core of inviscid neutron stars. Using a chiral $SU(2)_f$ sigma model, we construct isentropic temperature profiles for hot and dense matter and find that the frequency of the global core $g$-mode's dependence on temperature is governed by the nuclear symmetry energy slope parameter $L$. As a result, the $g$-mode frequency of a warm neutron star can be either higher or lower than that of its cold counterpart, depending on $L$. Our results highlight the interplay of thermal effects and composition gradients, and demonstrate the potential of neutron star $g$-mode observations to constrain the density dependence of the symmetry energy.

astro-ph.HE↗

arXiv2025

$g$-mode oscillations of dark matter admixed neutron stars

We investigate $g$-mode oscillations in dark matter admixed neutron stars employing a relativistic mean field model to describe hadronic matter and a model for self-interacting fermionic dark matter motivated by the neutron decay anomaly. Following the construction of such admixed configurations, we derive the equilibrium and adiabatic speeds of sound therein, leading to a computation of the star's $g$-mode spectrum in the Cowling approximation. In particular, we explore the effect of dark matter self-interaction, the nucleon effective mass and dark matter fraction on the principal $g$-mode frequency, and its first overtone. We show that the effect on $g$-mode frequency depends predominantly on the dark matter fraction, and demonstrate an equation of state-independent constraint for the latter. Prospects of identifying the presence of dark matter in neutron stars using $g$-mode are discussed.

gr-qc↗

arXiv2025

Effects of a Brueckner-Hartree-Fock-corrected effective mass on speed of sound, conformality, and observables of dark matter-admixed neutron stars

We construct an equation of state describing cold and dense matter in the core of neutron stars which includes an admixture of fermionic dark matter and incorporates nucleon effective masses derived from the relativistic Brueckner-Hartree-Fock (BHF) many-body approach within a relativistic mean-field model. Such a BHF-informed mixed-model approach increases stellar compactness, with mass-radius configurations which are consistent with smaller, lighter pulsars. The model displays the expected non-monotonic behavior of sound speed hinted at by neutron star data, and is closer to the conformal bound at maximum mass. We find that the model displays tension with bounds on heavier pulsars, suggesting that the hypothesis of an aggregated dark component in neutron stars needs further critical study.

hep-ph↗

arXiv2023

Quark clusters, QCD vacuum and the cosmological 7Li, Dark Matter and Dark Energy problems

We propose a non-exotic electromagnetic solution (within the standard model of particle physics) to the cosmological 7Li problem based upon a narrow 2 MeV photo-emission line from the decay of light Glueballs (LGBs). These LGBs form within color superconducting, tens of Fermi in size, quark clusters (SQCs) in the radiation-dominated post-BBN epoch. The mono-chromatic line from the LGB -> gamma+gamma decay reduces Big-Bang nucleosynthesis (BBN) 7Be by 2/3 without affecting other abundances or CMB physics, provided the combined mass of the SQCs is greater than the total baryonic mass in the Universe. Following the LGB emission, the in-SQC Quantum-ChromoDynamics (QCD) vacuum becomes unstable and "leaks" (via quantum tunnelling) into the external space-time (trivial) vacuum inducing a decoupling of SQCs from hadrons. In seeking a solution to the 7Li problem, we uncovered a solution which also addresses the dark energy (DE) and dark matter (DM) problem making these critical problems intertwined in our model. Being colorless, charge neutral, optically thin and transparent to hadrons, SQCs interact only gravitationally making them a viable CDM candidate. The quantum tunnelling of the in-SQC QCD vacuum to the trivial vacuum offers an explanation of DE in our model and allows for a cosmology which evolves into a LambdaCDM universe at low redshift with a possible resolution of the Hubble tension. Our model distinguishes itself by proposing that the QCD vacuum within SQCs possesses the ability to tunnel into the exterior trivial vacuum, resulting in the generation of DE. This implies the possibility that DM and hadrons might represent distinct phases of quark matter within QCD, characterized by different vacuum properties. We discuss SQC formation in heavy-ion collision experiments at moderate temperatures and the possibility of detection of MeV photons from LGB -> gamma+gamma.

astro-ph.CO↗

arXiv2022

g-mode Oscillations in Neutron Stars with Hyperons

A common alternative to the standard assumption of nucleonic composition of matter in the interior of a neutron star is to include strange baryons, particularly hyperons. Any change in composition of the neutron star core has an effect on g-mode oscillations of neutron stars, through the compositional dependence of the equilibrium and adiabatic sound speeds. We study the core g-modes of a neutron star contaning hyperons, using a variety of relativistic mean field models of dense matter that satisfy observational constraints on global properties of neutron stars. Our selected models predict a sharp rise in the g-mode frequencies upon the onset of strange baryons. Should g-modes be observed in the near future, their frequency could be used to test the presence of hyperonic matter in the core of neutron stars.

nucl-th↗

arXiv2022

Temperature Effects on Core g-modes of Neutron Stars

Neutron stars provide a unique physical laboratory to study the properties of matter at high density. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be excited by tidal forces and couple to gravitational waves. We extend prior results for the g-mode spectrum of cold neutron star matter to temperatures that are expected to be achieved in neutron star mergers using a parameterization for finite-temperature effects recently proposed by Raithel, \"Ozel and Psaltis. We find that the g-modes of canonical mass neutron stars ($\approx$1.4$M_{\odot}$) are suppressed at high temperature, and core $g$-modes are supported only in the most massive ($\geq $2$M_{\odot}$) of hot neutron stars.

astro-ph.HE↗

arXiv2022

General relativistic treatment of $f$-mode oscillations of hyperonic stars

We present a systematic study of $f$-mode oscillations in neutron stars containing hyperons, extending recent results obtained within the Cowling approximation to linearized General Relativity. Employing a relativistic mean field model, we find that the Cowling approximation can overestimate the quadrupolar $f$-mode frequency of neutron stars by up to 30\% compared to the frequency obtained in the linearized general relativistic formalism. Imposing current astrophysical constraints, we derive updated empirical relations for gravitational wave asteroseismology. The frequency and damping time of quadrupole $f$-mode oscillations of hyperonic stars are found to be in the range of 1.47 - 2.45kHz and 0.13 - 0.51 sec respectively. Our correlation studies demonstrate that among the various parameters of the nucleonic and hyperonic sectors of the model, the nucleon effective mass shows the strongest correlation with mode characteristics and neutron star observables. Estimates for the detectability of $f$-modes in a transient burst of gravitational waves from isolated hyperonic stars is also provided.

astro-ph.HE↗

arXiv2022

Quasi-normal g-modes of neutron stars with quarks

Quasi-normal oscillation modes of neutron stars provide a means to probe their interior composition using gravitational wave astronomy. We compute the frequencies and damping times of composition-dependent core g-modes of neutron stars containing quark matter employing linearized perturbative equations of general relativity. We find that ignoring background metric perturbations due to the oscillating fluid, as in the Cowling approximation, underestimates the g-mode frequency by up to 10% for higher mass stars, depending on the parameters of the nuclear equation of state and how the mixed phase is constructed. The g-mode frequencies are well-described by a linear scaling with the central lepton (or combined lepton and quark) fraction for nucleonic (hybrid) stars. Our findings suggest that neutron stars with and without quarks are manifestly different with regards to their quasi-normal g-mode spectrum, and may thus be distinguished from one another in future observations of gravitational waves from merging neutron stars.

gr-qc↗

arXiv2021

Thermal Relaxation and Cooling of Quark Stars with a Strangelet Crust

In this article, we explore the cooling of isolated quark stars. These objects are structured of a homogeneous quark matter core and crusted by matter. To do this, we adopt two kinds of crust: (i) a crust made of purely nuclear matter following the Baym-Pethick-Sutherland (BPS) equation of state (EoS) and (ii) a crust made of nuggets of strange quark matter (strangelets). Both models have the same quark matter core described by the MIT bag model EoS. Our main purpose is to quantify the effects of a strangelet crust on the cooling and relaxation times of these strange stars. We also perform a thorough study of the thermal relaxation of quark stars, in which we have found that objects with a strangelet crust have a significantly different thermal relaxation time. Our study also includes the possible effects of color superconductivity in the quark core.

hep-ph↗

arXiv2021

$g$-modes of neutron stars with hadron-to-quark crossover transitions

We perform the first study of the principal core $g$-mode oscillation in hybrid stars containing quark matter, utilizing a crossover model for the hadron-to-quark transition inspired by lattice QCD. The ensuing results are compared with our recent findings of $g$-mode frequencies in hybrid stars with a first-order phase transition using Gibbs constructions. We find that models using Gibbs construction yield $g$-mode amplitudes and the associated gravitational energy radiated that dominate over those of the chosen crossover model owing to the distinct behaviors of the equilibrium and adiabatic sound speeds in the various models. Based on our results, we conclude that were $g$-modes to be detected in upgraded LIGO and Virgo detectors it would indicate a first-order phase transition akin to a Gibbs construction.

astro-ph.HE↗

arXiv2021

g-mode Oscillations in Hybrid Stars: A Tale of Two Sounds

We study the principal core g-mode oscillation in hybrid stars containing quark matter and find that they have an unusually large frequency range ($\approx$ 200 - 600 Hz) compared to ordinary neutron stars or self-bound quark stars of the same mass. Theoretical arguments and numerical calculations that trace this effect to the difference in the behaviour of the equilibrium and adiabatic sound speeds in the mixed phase of quarks and nucleons are provided. We propose that the sensitivity of core g-mode oscillations to non-nucleonic matter in neutron stars could be due to the presence of a mixed quark-nucleon phase. Based on our analysis, we conclude that for binary mergers where one or both components may be a hybrid star, the fraction of tidal energy pumped into resonant g-modes in hybrid stars can exceed that of a normal neutron star by a factor of 2-3, although resonance occurs during the last stages of inspiral. A self-bound star, on the other hand, has a much weaker tidal overlap with the g-mode. The cumulative tidal phase error in hybrid stars, $\Delta\phi\cong$ 0.5 rad, is comparable to that from tides in ordinary neutron stars, presenting a challenge in distinguishing between the two cases. However, should the principal g-mode be excited to sufficient amplitude for detection in a post-merger remnant with quark matter in its interior, its frequency would be a possible indication for the existence of non-nucleonic matter in neutron stars.

nucl-th↗

arXiv2019

The Structure of the Hadron-Quark Reaction Zone

Hadron-quark combustion in dense matter is a central topic in the study of phases in compact stars and their high-energy astrophysics. We critically review the literature on hadron-quark combustion, dividing them into a "first wave" that treats the problem as a steady-state burning with or without constraints of mechanical equilibrium, and a "second wave" which uses numerical techniques to resolve the burning front and solves the underlying Partial Differential Equations for the chemistry of the burning front under less restrictive conditions. We detail the inaccuracies that the second wave amends over the first wave, and highlight crucial differences between various approaches in the second wave. We also include results from time-dependent simulations of the reaction zone that include a hadronic EOS, neutrinos, and self-consistent thermodynamics without using parameterized shortcuts.

nucl-th↗

arXiv2018

Lifting the Veil on Quark Matter in Compact Stars with Core g-mode Oscillations

Compact stars containing quark matter may masquerade as neutron stars in the range of measured mass and radius, making it difficult to draw firm conclusions on the phase of matter inside the star. The sensitivity of core $g$-mode oscillations to the presence of a mixed phase may alleviate this difficulty. In hybrid stars that admit quark matter in a mixed phase, the $g$-mode frequency rises sharply due to a marked decrease in the equilibrium sound speed. Resonant excitation of $g$-modes can leave an imprint on the waveform of coalescing binary compact stars. We present some analytic and numeric results to assess the sensitivity displayed by $g$-mode oscillations to quark matter in a homogeneous or mixed phase and find that the $g$-mode can probe the fraction of quark matter inside the star.

nucl-th↗

arXiv2018

Camouflage of the Phase Transition to Quark Matter in Neutron Stars

It has been known for some time that compact stars containing quark matter can masquerade as neutron stars in the range of measured mass and radius, making it difficult to draw firm conclusions on the phases of matter present inside the star. Using the vector-enhanced Bag model (vBag), we examine mass-radius and mass-compactness relations with Maxwell and Gibbs construction for hybrid stars with transitions from nuclear matter to two or three-flavor quark matter, including sequential transitions. Not only can stable hybrid stars with either two or three-flavor quark matter mimic neutron stars (the traditional masquerade), it appears as well difficult to distinguish two-flavor from three-flavor quark matter even in cases where a phase transition can be said to have occurred, as in the presence of a distinct kink in the mass-radius relation. Furthermore, allowing for sequential flavor transitions, we find that the transition into an unstable branch can be caused by either a transition from a nuclear to unstable quark matter or the sequential transition from nuclear to stable but "masquerading" two-flavor to unstable three-flavor quark matter. Addressing chiral restoration as well as quark deconfinement in a model of the phase transition, as the vBag does, adds further flexibility to the high-density equation of state, motivating caution in using even high-precision $M$-$R$ data to draw firm conclusions on the nature of phases and phase transitions in neutron stars.

nucl-th↗

arXiv2018

Interface effects of strange quark matter with density dependent quark masses

We study the interface effects in strangelets adopting mean-field approximation (MFA). Based on an equivparticle model, the linear confinement and leading-order perturbative interactions are included with density-dependent quark masses. By increasing the confinement strength, the surface tension and curvature term of strange quark matter (SQM) become larger, while the perturbative interaction does the opposite. For those parameters constrained according to the 2$M_\odot$ strange star, the surface tension is $\sim$2.4 MeV/fm${}^2$, while unstable SQM indicates a slightly larger surface tension. The obtained results are then compared with those predicted by the multiple reflection expansion (MRE) method. In contrast to the bag model case, it is found that MRE method overestimates the surface tension and underestimates the curvature term. To reproduce our results, the density of states in the MRE approach should be modified by proper damping factors.

hep-ph↗

arXiv2018

Hadron-Quark Combustion as a Nonlinear, Dynamical System

The hadron-quark combustion front is a system that couples various processes, such as chemical reactions, hydrodynamics, diffusion, and neutrino transport. Previous numerical work has shown that this system is very nonlinear, and can be very sensitive to some of these processes. In these proceedings, we contextualize the hadron-quark combustion as a nonlinear system, subject to dramatic feedback triggered by leptonic weak decays and neutrino transport.

nucl-th↗

arXiv2017

Non-radial Oscillation Modes of Compact Stars with a Crust

Oscillation modes of isolated compact stars can, in principle, be a fingerprint of the equation of state (EoS) of dense matter. We study the non-radial high-frequency l=2 spheroidal modes of neutron stars and strange quark stars, adopting a two-component model (core and crust) for these two types of stars. Using perturbed fluid equations in the relativistic Cowling approximation, we explore the effect of a strangelet or hadronic crust on the oscillation modes of strange stars. The results differ from the case of neutron stars with a crust. In comparison to fluid-only configurations, we find that a solid crust on top of a neutron star increases the p-mode frequency slightly with little effect on the f-mode frequency, whereas for strange stars, a strangelet crust on top of a quark core significantly increases the f-mode frequency with little effect on the p-mode frequency.

gr-qc↗