J. C. Niemeyer

Three-dimensional simulations of type Ia supernovae

We present the results of three-dimensional hydrodynamical simulations of the subsonic thermonuclear burning phase in type Ia supernovae. The burning front model contains no adjustable parameters so that variations of the explosion outcome can be linked directly to changes in the initial conditions. In particular, we investigate the influence of the initial flame geometry on the explosion energy and find that it appears to be weaker than in 2D. Most importantly, our models predict global properties such as the produced nickel masses and ejecta velocities within their observed ranges without any fine tuning.

Delayed detonations in full-star models of type Ia supernova explosions

Aims. We present the first full-star three-dimensional explosion simulations of thermonuclear supernovae including parameterized deflagration-to-detonation transitions that occur once th e flame enters the distributed burning regime. Methods. Treating the propagation of both the deflagration and the det onation waves in a common front-tracking approach, the detonation is prevented from crossing ash regions. Results. Our criterion triggers the detonation wave at the outer edge of the deflagration flame and consequently it has to sweep around the complex structure and to compete with expansion. Despite the impeded detonation propagation, the obtained explosions show reasonable agreement with global quantities of observed Type Ia supernovae. By igniting the flame in di fferent numbers of kernels around the center of the exploding white dwarf, we set up three different models shifting the emphasis from the deflagration phase to the detonation phase. The resulting explosion energies and iron group element productions cover a large part of the diversity of Type Ia supernovae. Conclusions. Flame-driven deflagration-to-detonation transitions, if hypothetical, remain a possibility deserving further inve stigation.

Inflation with a Planck-scale frequency cutoff

The implementation of a Planck-scale high frequency and short wavelength cutoff in quantum theories on expanding backgrounds may have potentially nontrivial implications, such as the breaking of local Lorentz invariance and the existence of a yet unknown mechanism for the creation of vacuum modes. In scenarios where inflation begins close to the cutoff scale, these effects could have observable consequences as trans-Planckian modes are redshifted to cosmological scales. In close analogy with similar studies of Hawking radiation, a simple theory of a minimally coupled scalar field in de Sitter space is studied, with a high frequency cutoff imposed by a nonlinear dispersion relation. Under certain conditions the model predicts deviations from the standard inflationary scenario. We also comment on the difficulties in generalizing fluid models of Hawking radiation to cosmological space-times.

Transplanckian dispersion and scale invariance of inflationary perturbations

We question the insensitivity of the predictions of inflationary models with respect to modifications of Planck energy physics. The modification we consider consists in replacing the usual dispersion relation by nonlinear ones. This way of addressing the problem has recently received attention and contradictory results were found. Our main result is to show that the adiabaticity of the mode propagation and the separation of two scales of interest, the Planck scale and the cosmological horizon scale, are sufficient conditions for the predictions to be unchanged. We then show that almost all models satisfy the first condition when the second is met. Therefore the introduction of a nonlinear dispersion is unlikely to have any discernable effects on the power spectrum of cosmological perturbations.

Refined numerical models for multidimensional type Ia supernova simulations

Following up on earlier work on this topic (Reinecke et al. 1999b,a), we present an improved set of numerical models for simulations of white dwarfs exploding as type Ia supernovae (SNe Ia). Two-dimensional simulations were used to test the reliability and numerical robustness of these algorithms; the results indicate that integral quantities like the total energy release are insensitive to changes of the grid resolution (above a certain threshold), which was not the case for our former code. The models were further enhanced to allow fully three-dimensional simulations of SNe Ia. A direct comparison of a 2D and a 3D calculation with identical initial conditions shows that the explosion is considerably more energetic in three dimensions; this is most likely caused by the assumption of axisymmetry in 2D, which inhibits the growth of flame instabilities in the azimuthal direction and thereby decreases the flame surface.

A localised subgrid scale model for fluid dynamical simulations in astrophysics - II. Application to type Ia supernovae

The dynamics of the explosive burning process is highly sensitive to the flame speed model in numerical simulations of type Ia supernovae. Based upon the hypothesis that the effective flame speed is determined by the unresolved turbulent velocity fluctuations, we employ a new subgrid scale model wh ich includes a localised treatment of the energy transfer th rough the turbulence cascade in combination with semi-statistical closures for the dissipation and non-local transport of t urbulence energy. In addition, subgrid scale buoyancy effects are included. In the limit of negligible energy transfe r and transport, the dynamical model reduces to the Sharp-Wheeler relation. According to our findings, the Sharp-Wheeler relation is insu ffcient to account for the complicated turbulent dynamics of flames i n thermonuclear supernovae. The application of a co-moving grid technique enables us to achieve very high spatial resolution in the burning region. Turbulence is produced mostly at the flame surface and in the interior ash regions. Consequently, ther e is a pronounced anisotropy in the vicinity of the flame front s. The localised subgrid scale model predicts significantly enhan ced energy generation and less unburnt carbon and oxygen at low velocities compared to earlier simulations.

Multi-spot ignition in type Ia supernova models

We present a systematic survey of the capabilities of type Ia supernova explosion models starting from a number of flame seeds distributed around the center of the white dwarf star. To this end we greatly improved the resolution of the numerical simulations in the initial stages. This novel numerical approach facilitates a detailed study of multi-spot ignition scenarios with up to hundreds of ignition sparks. Two-dimensional simulations are shown to be inappropriate to study the effects of initial flame configurations. Based on a set of three-dimensional models, we conclude that multi-spot ignition scenarios may improve type Ia supernova models towards better agreement with observations. The achievable effect reaches a maximum at a limited number of flame ignition kernels as shown by the numerical models and corroborated by a simple dimensional analysis.

Minimal modifications of the primordial power spectrum from an adiabatic short distance cutoff

As a simple model for unknown Planck scale physics, we assume that the quantum modes responsible for producing primordial curvature perturbations during inflation are placed in their instantaneous adiabatic vacuum when their proper momentum reaches a fixed high energy scale M. The resulting power spectrum is derived and presented in a form that exhibits the amplitude and frequency of the superimposed oscillations in terms of

On the small-scale stability of thermonuclear flames in Type Ia supernovae

H/M

The cellular burning regime in type Ia supernova explosions : II. Flame propagation into vortical fuel

and the slow roll parameter