arXiv subjects

J. C. Niemeyer

Publications and source records attributed to J. C. Niemeyer.

60 records · Page 4Linked to original sources

arXiv1998

Numerical investigation of the threshold for primordial black hole formation

First results of a numerical investigation of primordial black hole formation in the radiation dominated phase of the Early Universe are presented. The simulations follow the gravitational collapse of three different families of high-amplitude density fluctuations imposed at the time of horizon crossing. The threshold for black hole formation, δ_{c} \approx 0.7, is found to be nearly identical for all perturbation families if the control parameter, δ, is chosen as the total excess mass within the initial horizon volume. Furthermore, we demonstrate that the scaling of black hole mass with distance from the formation threshold, known to occur in near-critical gravitational collapse, applies to primordial black hole formation.

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arXiv1998

Near-Critical Gravitational Collapse and the Initial Mass Function of Primordial Black Holes

The recent discovery of critical phenomena arising in gravitational collapse near the threshold of black hole formation is used to estimate the initial mass function of primordial black holes (PBHs). It is argued that the universal scaling relation between black hole mass and initial perturbation found for a variety of collapsing space-times also applies to PBH formation, indicating the possibility of the formation of PBHs with masses much smaller than one horizon mass. Owing to the natural fine-tuning of initial conditions by the exponential decline of the probability distribution for primordial density fluctuations, sub-horizon mass PBHs are expected to form at all epochs. This result suggests that the constraints on the primordial fluctuation spectrum based on the abundance of PBHs at different mass scales may have to be revisited.

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arXiv1998

Phases of a Type Ia supernova explosion

In the framework of the Chandrasekhar mass white dwarf model for Type Ia supernovae, various stages of the explosion are described in terms of the burning regimes of the thermonuclear flame front. In the early flamelet regime following the ``smoldering'' phase prior to the explosion, the flame is sufficiently thin and fast to remain laminar on small scales. As the white dwarf density declines, the thermal flame structure becomes subject to penetration by turbulent eddies, and it enters the ``distributed burning'' regime. A specific control parameter for this transition is proposed. Furthermore, we outline an argument for the coincidence of the transition between burning regimes with the onset of a deflagration-detonation-transition (DDT) in the late phase of the explosion.

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arXiv1997

Numerical Investigation of Scaling Properties of Turbulent Premixed Flames

Gibson scaling and related properties of flame-surface geometry in turbulent premixed combustion are demonstrated using a novel computational model, Deterministic Turbulent Mixing (DTM). In DTM, turbulent advection is represented by a sequence of maps applied to the computational domain. The structure of the mapping sequence incorporates pertinent scaling properties of the turbulent cascade. Here, combustion in Kolmogorov turbulence (kinetic-energy cascade) and in Bolgiano-Obukhov convective turbulence (potential-energy cascade) is simulated. Implications with regard to chemical flames and astrophysical (thermonuclear) flames are noted.

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arXiv1996

The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs

The flame born in the deep interior of a white dwarf that becomes a Type Ia supernova is subject to several instabilities. We briefly review these instabilities and the corresponding flame acceleration. We discuss the conditions necessary for each of the currently proposed explosion mechanisms and the attendant uncertainties. A grid of critical masses for detonation in the range $10^7$ - $2 \times 10^9$ g cm$^{-3}$ is calculated and its sensitivity to composition explored. Prompt detonations are physically improbable and appear unlikely on observational grounds. Simple deflagrations require some means of boosting the flame speed beyond what currently exists in the literature. ``Active turbulent combustion'' and multi-point ignition are presented as two plausible ways of doing this. A deflagration that moves at the ``Sharp-Wheeler'' speed, $0.1 g_{\rm eff} t$, is calculated in one dimension and shows that a healthy explosion is possible in a simple deflagration if the front moves with the speed of the fastest floating bubbles. The relevance of the transition to the ``distributed burning regime'' is discussed for delayed detonations. No model emerges without difficulties, but detonation in the distributed regime is plausible, will produce intermediate mass elements, and warrants further study.

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arXiv1996

Off-Center Deflagrations In Chandrasekhar Mass SN Ia Models

A series of two dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandraskhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star, and continues until the density in the burning regions has declined to about 10$^7$ g cm$^{-3}$, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modelling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate 1) a centrally ignited deflagration, 2) off-center ignition at a single ``point", and 3) simultaneous off-center ignition at five ``points". We find that the amount of $^{56}$Ni that is produced and other observable properties depend sensitively upon how the fuel is ignited.

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