Gravitational Collapse Of Massive Stars Research Articles

Charged gravastar model in Rastall theory of gravity

Gravastars are considered as exotic compact objects, may be found at the end of gravitational collapse of massive stars, to resolve the complexities of black holes. In this paper, we analyse the role of charge on the possible formation of an isotropic spherically symmetric gravastar configuration in the framework of Rastall gravity. Gravastar contains three distinct layers viz. (i) Interior region characterised by the equation of state, p=−ρ, which defines the repulsive outward pressure in the radial direction at all points on the thin shell, (ii) Thin shell contains an ultra-relativistic stiff fluid, which is denoted by the equation of state p=ρ following Zel'dovich's criteria for a cold baryonic universe, and can withstand the repulsive pressure exerted by the interior region, and (iii) Exterior region which is the vacuum space-time represented by the Reissner-Nordström solution. Following above specifications, we construct and analyse charged gravastar model in the framework of Rastall gravity, which represents several salient features. The basic physical attributes, viz. proper length, energy, entropy and equation of state parameter of the shell are investigated. In this model, it is interesting to note that for a large radius of the hypersurface (R), the equation of state parameter of the thin shell corresponds to a dark energy equation of state with W(R)→−1. However, for small value of R, W(R)→0, defines dust shell. The stability of the model is ensured through the study of gravitational surface redshift and maximisation of shell entropy within the framework of Rastall theory of gravity.

Nonlinear Evolution of Fast Collective Neutrino Oscillations in Core-Collapse Supernovae

According to one of the most promising supernova theories, the neutrino-heating mechanism, neutrinos are responsible for transferring the energy released during the gravitational collapse of massive stars to their surroundings. If neutrino flavors are converted fast in the cores, the efficiency of neutrino heating is enhanced and can change the dynamics of the shock wave in supernovae. In this article, we investigate the dynamics of fast neutrino flavor conversions with collisions under energy-dependent treatment in detail. For the first time, we use a realistic initial condition, which is taken from the results of the self-consistent, realistic Boltzmann simulations in two spatial dimensions under axisymmetry. We report that the neutrino flavor conversion will be significantly enhanced if the energy-dependent collision term is considered in the neutrino transport equation which has been ignored in previous studies. We present the preliminary results of our investigation here, which are consistent with other studies which were conducted under simple treatments. It is believed that such findings may have an impact on the explosion mechanism of core-collapse supernovae, nucleosynthesis and neutrino astronomy.

Nonlinear Evolution of Fast Collective Neutrino Oscillations in Core-collapse Supernovae

According to one of the most promising supernova theories, the neutrino-heating mechanism, neutrinos are responsible for transferring the energy released during the gravitational collapse of massive stars to their surroundings. If neutrino flavors are converted fast in the cores, the efficiency of neutrino heating is enhanced and can change the dynamics of the shock wave in supernovae. In this article, we investigate the dynamics of fast neutrino flavor conversions with collisions under energy-dependent treatment in detail. For the first time, we use a realistic initial condition, which is taken from the results of the self-consistent, realistic Boltzmann simulations in two spatial dimensions under axisymmetry. We report that the neutrino flavor conversion will be significantly enhanced if the energy-dependent collision term is considered in the neutrino transport equation which has been ignored in previous studies. We present the preliminary results of our investigation here, which are consistent with other studies which were conducted under simple treatments. It is believed that such findings may have an impact on the explosion mechanism of core-collapse supernovae, \mbox{nucleosynthesis and neutrino astronomy.

Cooling of neutron stars with quark-hadron continuity

Neutron stars are high-density objects formed by the gravitational collapse of massive stars, and the whole star can be likened to a giant nucleus. The interior of a neutron star is considered to contain exotic particles and states which do not appear in a normal nucleus. The internal states are constrained by observations of masses and radii via the equation of state of highly dense nuclear matter. Within these constraints, a variety of exotic states have been discussed. The internal state of neutron stars is closely related to its neutrino emission process, which cools the star from the inside. This effect can be compared with observations of the surface temperature of neutron stars. However, despite the wide range of observations of neutron stars, the nature of the neutron star matter remains uncertain. We consider quark matter as an exotic state and perform cooling calculations for neutron stars, incorporating the effects of nucleon superfluidity and quark colour superconductivity.We take into account the “quark-hadron continuity”, in which the neutron superfluidity is succeeded by thedquark pairing. Furthermore, we obtained the range of the neutron star cooling curve, taking into account the difference in surface temperature due to the composition of the surface layer. We found that the existence of quark matter causes strong neutrino emission from quarks, which is moderately suppressedbysuperfluidity and superconductivity, and canexplain the cold surface temperature of neutron stars.

Review of low energy nuclear astrophysics

In this review of Low Energy Nuclear Astrophysics, we review the nuclear energy and evolution of the sun. Then we review the estimate of dark matter from galaxy rotation. Next, we discuss neutron star formation from the gravitational collapse of massive stars which produces neutron stars and their velocity. Then the origin of supernovae, the estimate of dark energy from supernovae acceleration, and experiments related to low energy nuclear astrophysics are reviewed.

Imprints of the Janis-Newman-Winicour spacetime on observations related to shadow and accretion

The final fate of gravitational collapse of massive stars has been a subject of interest for a long time since such a collapse may lead to black holes and naked singularities alike. Since, the formation of naked singularities is forbidden by the cosmic censorship conjecture, exploring their observational differences from black holes may be a possible avenue to search for these exotic objects. The simplest possible naked singularity spacetime emerges from the Einstein massless scalar field theory with the advantage that it smoothly translates to the Schwarzschild solution by the variation of the scalar charge. This background, known as the Janis-Newman-Winicour spacetime is the subject of interest in this work. %A further advantage of working with this spacetime is that it smoothly translates to the Schwarzschild solution by the variation of the scalar charge. We explore electromagnetic observations around this metric which involves investigating the characteristics of black hole accretion and shadow. We compute the shadow radius in this spacetime and compare it with the image of M87*, recently released by the Event Horizon Telescope Collaboration. Similarly, we derive the expression for the luminosity from the accretion disk and compare it with the observed optical luminosity of eleven Palomar Green quasars. Our analysis indicates that the shadow of M87* and the quasar optical data consistently favor the Schwarzschild background over the Janis-Newman-Winicour spacetime. The implications of this result are discussed.

Influence of Density Dependence of Symmetry Energy in Hot and Dense Matter for Supernova Simulations

Abstract We study the influence of density-dependent symmetry energy at high densities in simulations of core-collapse supernovae, black hole formation, and proto–neutron star cooling by extending the relativistic mean field (RMF) theory used for the Shen equation-of-state (EOS) table. We adopt the extended RMF theory to examine the density dependence of the symmetry energy with a small value of the slope parameter L, while the original properties of the symmetric nuclear matter are unchanged. In order to assess matter effects at high densities, we perform numerical simulations of gravitational collapse of massive stars adopting the EOS table at high densities beyond 1014 g cm−3 with the small L value, which is in accord with the experimental and observational constraints, and compare them with the results obtained by using the Shen EOS. Numerical results for 11.2 and 15 M ⊙ stars exhibit minor effects around the core bounce and in the following evolution for 200 ms. Numerical results for 40 and 50 M ⊙ stars reveal a shorter duration toward the black hole formation with a smaller maximum mass for the small-L case. Numerical simulations of proto–neutron star cooling over 10 s through neutrino emissions demonstrate increasing effects of the symmetry energy at high densities. Neutrino cooling drastically proceeds in a relatively long timescale with high luminosities and average energies with the small symmetry energy. Evolution toward the cold neutron star is affected because of the different behavior of neutron-rich matter, while supernova dynamics around core bounce remains similar in less neutron-rich environments.

Determination of properties of protoneutron stars toward black hole formation via gravitational wave observations

We examine the frequencies of the fundamental ($f$-) and the gravity ($g_i$-) mode in gravitational waves from accreting protoneutron stars (PNSs) toward black hole formation. For this purpose, we analyze numerical results of gravitational collapse of massive stars for two different progenitors with three equations of state. We adopt profiles of central objects obtained from the numerical simulations by solving general relativistic neutrino-radiation hydrodynamics under the spherical symmetry. Using a series of snapshots as a static configuration at each time step, we solve the eigenvalue problem to determine the specific frequencies of gravitational waves from the evolving PNSs with accretion from core bounce to black hole formation by the relativistic Cowling approximation. We find that the frequency of the $f$-mode gravitational waves can be expressed as a function of the average density of the PNS at each time step almost independently of the progenitor models in accord with the previous studies on ordinary PNSs. We also find that the ratio of the $g_1$-mode frequency to the $f$-mode frequency is insensitive to the progenitor, but strongly depends on the compactness of the PNSs. Having a simultaneous observation of the $f$- and $g_1$-mode gravitational waves, it would provide the information of the average density and compactness of PNS, which can reconstruct the evolution of mass and radius of the PNSs as a constraint on the equation of state at high densities. Furthermore, we show that the highest average density of the PNS with the maximum mass can be estimated with the detection of the $f$-mode gravitational waves together with the help of the neutrino observations.

Where Does the Physics of Extreme Gravitational Collapse Reside?

The gravitational collapse of massive stars serves to manifest the most severe deviations of general relativity with respect to Newtonian gravity: the formation of horizons and spacetime singularities. Both features have proven to be catalysts of deep physical developments, especially when combined with the principles of quantum mechanics. Nonetheless, it is seldom remarked that it is hardly possible to combine all these developments into a unified theoretical model, while maintaining reasonable prospects for the independent experimental corroboration of its different parts. In this paper we review the current theoretical understanding of the physics of gravitational collapse in order to highlight this tension, stating the position that the standard view on evaporating black holes stands for. This serves as the motivation for the discussion of a recent proposal that offers the opposite perspective, represented by a set of geometries that regularize the classical singular behavior and present modifications of the near-horizon Schwarzschild geometry as the result of the propagation of non-perturbative ultraviolet effects originated in regions of high curvature. We present an extensive exploration of the necessary steps on the explicit construction of these geometries, and discuss how this proposal could change our present understanding of astrophysical black holes and even offer the possibility of detecting genuine ultraviolet effects in gravitational-wave experiments.

Black holes turn white fast, otherwise stay black: no half measures

Recently, various authors have proposed that the first ultraviolet effect on the gravitational collapse of massive stars to black holes is the transition between a black-hole geometry and a white-hole geometry, though their proposals are radically different in terms of their physical interpretation and characteristic time scales [1,2]. Several decades ago, it was shown by Eardley that white holes are highly unstable to the accretion of small amounts of matter, being rapidly turned into black holes [3]. Studying the crossing of null shells on geometries describing the black-hole to white-hole transition, we obtain the conditions for the instability to develop in terms of the parameters of these geometries. We conclude that transitions with long characteristic time scales are pathologically unstable: occasional perturbations away from the perfect vacuum around these compact objects, even if being imperceptibly small, suffocate the white hole explosion. On the other hand, geometries with short characteristic time scales are shown to be robust against perturbations, so that the corresponding processes could take place in real astrophysical scenarios. This motivates a conjecture about the transition amplitudes of different decay channels for black holes in a suitable ultraviolet completion of general relativity.

Collapsing spherical stars inf(R)gravity

We perform a careful investigation of the problem of physically realistic gravitational collapse of massive stars in f(R)-gravity. We show that the extra matching conditions that arise in the modified gravity imposes strong constraints on the stellar structure and thermodynamic properties. In our opinion these constraints are unphysical. We prove that no homogeneous stars with non-constant Ricci scalar can be matched smoothly with a static exterior for any nonlinear function f(R). Therefore, these extra constraints make classes of physically realistic collapse scenarios in general relativity, non-admissible in these theories. We also find an exact solution for an inhomogeneous collapsing star in the Starobinski model that obeys all the energy and matching conditions. However, we argue that such solutions are fine-tuned and unstable to matter perturbations. Possible consequences on black hole physics and the cosmic censorship conjecture are also discussed.

Naked singularities as particle accelerators. II.

We generalize here our earlier results on particle acceleration by naked singularities. We showed recently [M. Patil and P. S. Joshi, Phys. Rev. D 82, 104049 (2010).] that the naked singularities that form due to the gravitational collapse of massive stars provide a suitable environment where particles could get accelerated and collide at arbitrarily high center-of-mass energies. However, we focused there only on the spherically symmetric gravitational collapse models, which were also assumed to be self-similar. In this paper, we broaden and generalize the result to all gravitational collapse models leading to the formation of a naked singularity as the final state of collapse, evolving from a regular initial data, without making any prior restrictive assumptions about the spacetime symmetries such as above. We show that, when the particles interact and collide near the Cauchy horizon, the energy of collision in the center-of-mass frame will be arbitrarily high, thus offering a window to the Planck scale physics. We also consider the issue of various possible physical mechanisms of generation of such very high-energy particles from the vicinity of naked singularity. We then construct a model of gravitational collapse to a timelike naked singularity to demonstrate the working of these ideas, where the pressure is allowed to be negative, but the energy conditions are respected. We show that a finite amount of mass-energy density has to be necessarily radiated away from the vicinity of the naked singularity as the collapse evolves. Therefore, the nature of naked singularities, both at the classical and quantum level, could play an important role in the process of particle acceleration, explaining the occurrence of highly energetic outgoing particles in the vicinity of the Cauchy horizon that participate in extreme high-energy collisions.

THREE-DIMENSIONAL MAGNETOHYDRODYNAMICAL SIMULATIONS OF GRAVITATIONAL COLLAPSE OF A 15 M ☉ STAR

We introduce our newly developed two different, three dimensional magneto hydrodynamical codes in detail. One of our codes is written in the Newtonian limit (NMHD) and the other is in the fully general relativistic code (GRMHD). Both codes employ adaptive mesh refinement and, in GRMHD, the metric is evolved with the "Baumgarte-Shapiro-Shibata-Nakamura" formalism known as the most stable method at present. We did several test problems and as for the first practical test, we calculated gravitational collapse of a $15M_\odot$ star. Main features found from our calculations are; (1) High velocity bipolar outflow is driven from the proto-neutronstar and moves through along the rotational axis in strongly magnetized models; (2) A one-armed spiral structure appears which is originated from the low-$|T/W|$ instability; (3) By comparing GRMHD and NMHD models, the maximum density increases about $\sim30%$ in GRMHD models due to the stronger gravitational effect. These features agree very well with previous studies and our codes are thus reliable to numerical simulation of gravitational collapse of massive stars.

GRAVITATIONAL COLLAPSE OF MASSIVE STARS AS A PROBE INTO HOT DENSE MATTER

Nuclear physics is an indispensable input for the investigation of high energy astrophysical phenomena involving compact objects. In this paper I take a gravitational collapse of massive stars as an example and show how the macroscopic dynamics is influenced by the properties of nuclei and nuclear matter. I will discuss two topics that are rather independent of each other. The first one is the interplay of neutrino-nuclei inelastic scatterings and the standing accretion shock instability in the core of core collapse supernovae and the second is concerning the neutrino emissions from black hole formations and their dependence on the equation of state at very high densities. In the latter, I will also demonstrate that future astronomical observations might provide us with valuable information on the equation of state of hot dense matter.

EQUATION OF STATE OF DENSE MATTER FOR CORE-COLLAPSE SUPERNOVAE AND NEUTRINO BURSTS

We report the recent developments on the tables of equation of state for dense matter and their influence on core-collapse supernovae and associated neutrino emissions. We study the gravitational collapse of massive stars by the numerical simulations with the tables of equation of state recently developed in relativistic many body frameworks. I discuss whether the equation of state of dense matter can be probed by the properties of neutrino signals from black hole forming supernovae, being different from ordinary neutrino bursts from supernova explosions.

Stellar Collapse with Hadron-Quark Phase Transition of Hot and Dense Matter

We construct an equation of state (EOS) including the hadron-quark phase transition by the Gibbs conditions for finite temperature. We adopt the EOS based on the relativistic mean field theory for the hadronic phase and the MIT bag model of the deconfined 3-flavor strange quark matter for the quark phase. Moreover, the gravitational collapse of massive stars with several masses is computed using our EOS and we find that the phase transition makes the interval time from the bounce to the black hole formation, namely the duration time of the neutrino emission, shorter for the model with a bounce. This fact implies that we may be able to probe observationally the EOS of hot and dense matter in future.

Influence of Equation of State in Supernova Simulations: Neutrinos from Proto-Neutron Star and Black Hole Formation

We present the influence of equation of state in supernovae from gravitational collapse of massive stars. We examine the important roles of EOS in supernova explosions, protoneutron stars (or black holes) and supernova neutrinos, adopting the relativistic EOS table for supernova simulations and the conventional EOS. We discuss the density-temperature range which appears in supernova simulations to see whether exotic hadrons or quarks may appear in supernova physics.

Spectral decomposition of the perturbation response of the Schwarzschild geometry.

The radiative Green's function for the one-dimensional wave equation with the Regge-Wheeler and Zerilli potentials is formally constructed from recently developed analytic representations for generalized spheroidal wave functions, and decomposed into a convergent sum over quasinormal modes, an integral around a branch cut in the frequency domain, and a high-frequency remnant of the free-space propagator. This paper discusses the contribution to the time response made by the quasinormal modes and, at very late times, by the branch-cut integral. The initial-value problem is considered for source fields with both compact and extended radial dependences, and the problem of the formal divergence of the integrals of extended sources over quasinormal-mode wave functions is solved. The branch-cut integral produces a weak late-time radiative power-law decay tail that will characterize the astrophysically observed radiation spectrum for times subsequent to the exponential decay of the quasinormal ringing, when (ct-${r}_{\mathrm{*}}$)\ensuremath{\gg}2MG/${c}^{2}$ and (ct-${r}_{\mathrm{*}}$)/${r}_{\mathrm{*}}$\ensuremath{\ll}1. This radiative decay tail is shown to diminish to Price's nonradiative tail in the final limit ct/${r}_{\mathrm{*}}$\ensuremath{\gg}1. The method is applied to a characteristic-value problem used to model the gravitational collapse of massive stars, and to the small-body radial in-fall problem. The analysis presented is generalizable, through the Newman-Penrose formalism and Teukolsky's equations, to obtain the radiative Green's function for perturbations to the Kerr geometry.

Metastability of dense neutron matter

We formulate a theory of metastability of dense neutron matter with respect to the first-order phase transition to a pion-condensed state. The theory is based on the idea of nucleation of the pion-condensed phase in the metastable normal matter through the appearance of spontaneously growing drops of the new phase. Different paths leading from the false to the true ground state of dense neutron matter are considered. Also, both quantum fluctuations — that dominate at the low temperatures typical of neutron stars-and thermal fluctuations — that dominate during the gravitational collapse of massive stars — are considered. The calculations performed for realistic models of cold neutron matter yield an interval of metastability (on the time scale of the age of the universe) which is as large as half of that between the baryon density ρ N where the metastability starts and ρ c where the potential barrier (without surface effects) between the true and the false ground state vanishes. Possible astrophysical implications of this finding for neutron stars are discussed. In the case of the pion condensation in hot neutron matter ( T ~ 5 MeV) the region of metastability (on the time scale of the gravitational collapse of a massive star) is much narrower than in cold neutron matter. Consequently additional energy release and entropy generation from the first-order phase transition in hot supercompressed matter are found to be negligible. The theory outlined here can be used for the description of other phase transitions in dense nuclear matter, such as the transition to the quark phase for instance.

Lepton number violation, Majorana neutrinos, and supernovae

Weak interaction theories with Majorana masses for neutrinos have lepton number violating reactions. We consider gravitational collapse of massive stars in such a model and find that if the lepton nonconserving reactions occur, the dynamics of collapse will be drastically different from the low-entropy collapse that follows from the usual assumption of lepton number conservation.