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Results for “GRAVITATIONAL COLLAPSE”

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1965

Gravitational collapse.

Gravitational collapse as possible energy source for quasi-stellar sources, including discussion of radio sources and superdense stars

QUASI-STELLAR RADIO SOURCE /QSO/↗
DOE OSTI2014

HYPERCRITICAL ACCRETION, INDUCED GRAVITATIONAL COLLAPSE, AND BINARY-DRIVEN HYPERNOVAE

The induced gravitational collapse (IGC) paradigm has been successfully applied to the explanation of the concomitance of gamma-ray bursts (GRBs) with supernovae (SNe) Ic. The progenitor is a tight binary system composed of a carbon-oxygen (CO) core and a neutron star (NS) companion. The explosion of the SN leads to hypercritical accretion onto the NS companion, which reaches the critical mass, hence inducing its gravitational collapse to a black hole (BH) with consequent emission of the GRB. The first estimates of this process were based on a simplified model of the binary parameters and the Bondi-Hoyle-Lyttleton accretion rate. We present here the first full numerical simulations of the IGC phenomenon. We simulate the core-collapse and SN explosion of CO stars to obtain the density and ejection velocity of the SN ejecta. We follow the hydrodynamic evolution of the accreting material falling into the Bondi-Hoyle surface of the NS all the way up to its incorporation in the NS surface. The simulations go up to BH formation when the NS reaches the critical mass. For appropriate binary parameters, the IGC occurs in short timescales ∼10{sup 2}-10{sup 3} s owing to the combined effective action of the photon trapping and the neutrino cooling near the NS surface. We also show that the IGC scenario leads to a natural explanation for why GRBs are associated only with SNe Ic with totally absent or very little helium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

On the induced gravitational collapse scenario of gamma-ray bursts associated with supernovae

Following the induced gravitational collapse (IGC) paradigm of gamma-ray bursts (GRBs) associated with type Ib/c supernovae, we present numerical simulations of the explosion of a carbon–oxygen (CO) core in a binary system with a neutron-star (NS) companion. The supernova ejecta trigger a hypercritical accretion process onto the NS thanks to a copious neutrino emission and the trapping of photons within the accretion flow. We show that temperatures of 1–10 MeV develop near the NS surface, hence electron–positron annihilation into neutrinos becomes the main cooling channel leading to accretion rates of 10{sup −9}–10{sup −1} M{sub ⊙} s{sup −1} and neutrino luminosities of 10{sup 43}–10{sup 52} erg s{sup −1} (the shorter the orbital period the higher the accretion rate). We estimate the maximum orbital period, P{sub max}, as a function of the NS initial mass, up to which the NS companion can reach by hypercritical accretion the critical mass for gravitational collapse leading to black hole formation. We then estimate the effects of the accreting and orbiting NS companion onto a novel geometry of the supernova ejecta density profile. We present the results of a 1.4×10{sup 7} particle simulation which show that the NS induces accentuated asymmetries in the ejecta density around the orbital plane. We elaborate on the observables associated with the above features of the IGC process. We apply this framework to specific GRBs: we find that X-ray flashes (XRFs) and binary-driven hypernovae are produced in binaries with P>P{sub max} and P<P{sub max}, respectively. We analyze in detail the case of XRF 060218.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Gravitational collapse in f(R) theories

We study the gravitational collapse in modified gravitational theories. In particular, we analyze a general f(R) model with uniformly collapsing cloud of self-gravitating dust particles. This analysis shares analogies with the formation of large-scale structures in the early Universe and with the formation of stars in a molecular cloud experiencing gravitational collapse. In the same way, this investigation can be used as a first approximation to the modification that stellar objects can suffer in these modified theories of gravity. We study concrete examples, and find that the analysis of gravitational collapse is an important tool to constrain models that present late-time cosmological acceleration.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Gravitational collapse of dissipative fluid as a source of gravitational waves

Gravitational collapse of cylindrical anisotropic fluid has been considered in analogy with the work of Misner and Sharp. Using Darmois matching conditions, the interior cylindrical dissipative fluid (in the form of shear viscosity and heat flux) is matched to an exterior vacuum Einstein–Rosen space–time. It is found that on the bounding 3-surface the radial pressure of the anisotropic perfect fluid is linearly related to the shear viscosity and the heat flux of the dissipative fluid on the boundary. This non-zero radial pressure on the bounding surface may be considered as the source of gravitational waves outside the collapsing matter distribution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Numerical thermodynamic studies of classical gravitational collapse in 3+1 and 4+1 dimensions

Recent numerical work on the gravitational collapse of a five-dimemsional (5D) [4+1] Yang-Mills instanton has provided numerical evidence that the free energy F=E-TS=E/3 of a 5D Schwarzschild black hole of mass E can be obtained classically via the Lagrangian. Although there is no Hawking radiation, these numerical results suggest that the quantity TS has a classical meaning. We investigate this association for the physically relevant case of 3+1 dimensional collapse. We track numerically the negative of the total Lagrangian -L during the gravitational collapse of a massless scalar field to a Schwarzschild black hole in isotropic coordinates. We show that -L approaches the free energy F=E-TS=E/2 of a four-dimensional Schwarzschild black hole to within 5%. We also show that the matter contribution to the free energy tends towards zero so that the entropy at late stages of the collapse is gravitational in origin. The entropy S makes a negative contribution to the free energy and this feature is observed in our numerical simulation. There is a pronounced dip (negative contribution) in a thin slice just inside the event horizon precisely where the metric field is nonstationary. This is in accord with recent work suggesting black hole entropy is connected with the nonstationary phase space hidden behind the event horizon. We also obtain thermodynamic results for the 5D collapse of a massless scalar field which confirms that previous 5D results are universal and independent of the type of matter undergoing the collapse.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Gravitational collapse of dark energy field configurations and supermassive black hole formation

Dark energy is the dominant component of the total energy density of our Universe. The primary interaction of dark energy with the rest of the Universe is gravitational. It is therefore important to understand the gravitational dynamics of dark energy. Since dark energy is a low-energy phenomenon from the perspective of particle physics and field theory, a fundamental approach based on fields in curved space should be sufficient to understand the current dynamics of dark energy. Here, we take a field theory approach to dark energy. We discuss the evolution equations for a generic dark energy field in curved space-time and then discuss the gravitational collapse for dark energy field configurations. We describe the 3 + 1 BSSN formalism to study the gravitational collapse of fields for any general potential for the fields and apply this formalism to models of dark energy motivated by particle physics considerations. We solve the resulting equations for the time evolution of field configurations and the dynamics of space-time. Our results show that gravitational collapse of dark energy field configurations occurs and must be considered in any complete picture of our Universe. We also demonstrate the black hole formation as a result of the gravitational collapse of the dark energy field configurations. The black holes produced by the collapse of dark energy fields are in the supermassive black hole category with the masses of these black holes being comparable to the masses of black holes at the centers of galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE IMPACT OF THERMODYNAMICS ON GRAVITATIONAL COLLAPSE: FILAMENT FORMATION AND MAGNETIC FIELD AMPLIFICATION

Stars form by the gravitational collapse of interstellar gas. The thermodynamic response of the gas can be characterized by an effective equation of state. It determines how gas heats up or cools as it gets compressed, and hence plays a key role in regulating the process of stellar birth on virtually all scales, ranging from individual star clusters up to the galaxy as a whole. We present a systematic study of the impact of thermodynamics on gravitational collapse in the context of high-redshift star formation, but argue that our findings are also relevant for present-day star formation in molecular clouds. We consider a polytropic equation of state, P = k{rho}{sup {Gamma}}, with both sub-isothermal exponents {Gamma} < 1 and super-isothermal exponents {Gamma} > 1. We find significant differences between these two cases. For {Gamma} > 1, pressure gradients slow down the contraction and lead to the formation of a virialized, turbulent core. Weak magnetic fields are strongly tangled and efficiently amplified via the small-scale turbulent dynamo on timescales corresponding to the eddy-turnover time at the viscous scale. For {Gamma} < 1, on the other hand, pressure support is not sufficient for the formation of such a core. Gravitational contraction proceeds much more rapidly and the flow develops very strong shocks, creating a network of intersecting sheets and extended filaments. The resulting magnetic field lines are very coherent and exhibit a considerable degree of order. Nevertheless, even under these conditions we still find exponential growth of the magnetic energy density in the kinematic regime.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE INDUCED GRAVITATIONAL COLLAPSE OF A NEUTRON STAR TO A BLACK HOLE BY A TYPE Ib/c SUPERNOVA

It is understood that the supernovae (SNe) associated with gamma-ray bursts (GRBs) are of Type Ib/c. The temporal coincidence of the GRB and the SN continues to represent a major enigma of Relativistic Astrophysics. We elaborate here, from the earlier paradigm, that the concept of induced gravitational collapse is essential to explain the GRB-SN connection. The specific case of a close (orbital period <1 hr) binary system composed of an evolved star with a neutron star (NS) companion is considered. We evaluate the accretion rate onto the NS of the material expelled from the explosion of the core progenitor as a Type Ib/c SN and give the explicit expression of the accreted mass as a function of the nature of the components and binary parameters. We show that the NS can reach, in a few seconds, critical mass and consequently gravitationally collapse to a black hole. This gravitational collapse process leads to the emission of the GRB.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Preheating of the Intergalactic Medium by Gravitational Collapse and Ultraviolet Background

The preheating of the intergalactic medium by structure collapse and ultraviolet background (UVB) is investigated in cosmological hydrodynamical simulations. When gravitational collapse is the sole heating mechanism, we find that (1) 60% and 45% of the IGM are heated up to S > 8 and 17 keV cm{sup 2}, respectively, at z = 0, but the fractions drop rapidly to a few percent at z = 2; (2) the entropy of the circumhalo gas S{sub cir} is higher than the virial entropy for more than 75% of the halos with masses M<10{sup 11.5} M{sub ⊙} since z = 2, but the fraction higher than the entropy, S{sub pr}, required in the preventive model of galaxy formation is only 15%–20% for halos with M<10{sup 10.5} M{sub ⊙} at z = 0, and decreases as redshift increases; (3) assuming a metallicity of Z⩽0.03 Z{sub ⊙}, the fraction of halos whose circumhalo gas has a cooling time longer than the Hubble time t{sub cool,cir}>t{sub H} is merely 5%–10% at z≲0.5, and even less at z⩾1 for halos with M<10{sup 10.5} M{sub ⊙}; and (4) gas in the filaments undergoes the strongest preheating. Furthermore, we show that the UVB cannot enhance the fraction of the IGM with S>17 keV cm{sup 2}, but can increase the fraction of low-mass halos (<10{sup 10.5} M{sub ⊙}) having S{sub cir}>S{sub pr} to ∼70% at z = 0 and that having t{sub cool,cir}>t{sub H} to 15%–30% at z≲0.5. Our results indicate that preheating due to gravitational collapse and UVB is inadequate to fulfill the needs of the preventative model, especially for halos with 10{sup 10.5} M{sub ⊙}<M<10{sup 11.5} M{sub ⊙}. Nevertheless, these two mechanisms might cause large-scale galactic conformity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Gravitational wave extraction in simulations of rotating stellar core collapse

We perform simulations of general relativistic rotating stellar core collapse and compute the gravitational waves (GWs) emitted in the core-bounce phase of three representative models via multiple techniques. The simplest technique, the quadrupole formula (QF), estimates the GW content in the spacetime from the mass-quadrupole tensor only. It is strictly valid only in the weak-field and slow-motion approximation. For the first time, we apply GW extraction methods in core collapse that are fully curvature based and valid for strongly radiating and highly relativistic sources. These techniques are not restricted to weak-field and slow-motion assumptions. We employ three extraction methods computing (i) the Newman-Penrose (NP) scalar {Psi}{sub 4}, (ii) Regge-Wheeler-Zerilli-Moncrief master functions, and (iii) Cauchy-characteristic extraction (CCE) allowing for the extraction of GWs at future null infinity, where the spacetime is asymptotically flat and the GW content is unambiguously defined. The latter technique is the only one not suffering from residual gauge and finite-radius effects. All curvature-based methods suffer from strong nonlinear drifts. We employ the fixed-frequency integration technique as a high-pass waveform filter. Using the CCE results as a benchmark, we find that finite-radius NP extraction yields results that agree nearly perfectly in phase, but differ in amplitude by {approx}1%-7% at core bounce, depending on the model. Regge-Wheeler-Zerilli-Moncrief waveforms, while, in general, agreeing in phase, contain spurious high-frequency noise of comparable amplitudes to those of the relatively weak GWs emitted in core collapse. We also find remarkably good agreement of the waveforms obtained from the QF with those obtained from CCE. The results from QF agree very well in phase and systematically underpredict peak amplitudes by {approx}5%-11%, which is comparable to the NP results and is certainly within the uncertainties associated with core collapse physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

FORMATION OF KUIPER BELT BINARIES BY GRAVITATIONAL COLLAPSE

A large fraction of {approx}100 km class low-inclination objects in the classical Kuiper Belt (KB) are binaries with comparable masses and a wide separation of components. A favored model for their formation is that they were captured during the coagulation growth of bodies in the early KB. However, recent studies have suggested that large, {approx}>100 km objects can rapidly form in the protoplanetary disks when swarms of locally concentrated solids collapse under their own gravity. Here, we examine the possibility that KB binaries formed during gravitational collapse when the excess of angular momentum prevented the agglomeration of available mass into a solitary object. We find that this new mechanism provides a robust path toward the formation of KB binaries with observed properties, and can explain wide systems such as 2001 QW{sub 322} and multiples such as (47171) 1999 TC{sub 36}. Notably, the gravitational collapse is capable of producing {approx}100% binary fraction for a wide range of the swarm's initial angular momentum values. The binary components have similar masses ({approx}80% have a secondary-over-primary radius ratio >0.7) and their separation ranges from {approx}1000 to {approx}100,000 km. The binary orbits have eccentricities from e = 0 to {approx}1, with the majority having e < 0.6. The binary orbit inclinations with respect to the initial angular momentum of the swarm range from i = 0 to {approx}90{sup 0}, with most cases having i < 50{sup 0}. The total binary mass represents a characteristic fraction of the collapsing swarm's total initial mass, M{sub tot}, suggesting M{sub tot} equivalent to that of a radius {approx}100-250 km compact object. Our binary formation mechanism also implies that the primary and secondary components in each binary pair should have identical bulk composition, which is consistent with the current photometric data. We discuss the applicability of our results to the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius, and (90) Antiope binary systems.

79 ASTRONOMY AND ASTROPHYSICS↗