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F. K. Roepke

Publications and source records attributed to F. K. Roepke.

At least 109 records · Page 6Linked to original sources
arXiv2006

On the Mechanism of Type Ia Supernovae

Type Ia supernovae (SNe Ia) are one of the major tools to determine the cosmological parameters. Utilizing them as distance indicators, it is possible to geometrically survey the universe. To this end, the intrinsic scatter in the luminosities of these events needs to be calibrated using empirical relations between observables. A theoretical explanation for these relations is still lacking and can only be provided by a sound understanding of the mechanism of SNe Ia. Recently there has been significant progress in modeling SNe Ia. We report on numerical simulations of the explosion process, compare the results with observations of nearby SNe Ia, and discuss current uncertainties of the models. The presented simulations shed some light on the origin of the diversity of SNe Ia. Such simulations will pave the way towards an understanding of SN Ia diversities and correlations of their properties and ultimately provide a tool to validate the cosmological implications of SN Ia distance measurements.

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arXiv2006

Numerical Simulations of Type Ia Supernova Explosions

We present a systematic study of the diversity of three-dimensional deflagration simulations of Type Ia supernova explosions arising from variations of the initial parameters. By altering the carbon mass fraction, the central density, and the metallicity of the progenitor white dwarf star, we set up a grid of numerical explosion models. While changing the central density has the largest impact on the explosion energy, the largest variation in the 56Ni production is found by changing the metallicity of the models. Varying the carbon mass fraction hardly affects the 56Ni synthesized although it alters the energetics of the explosion. Possible consequences for the shape of light curves of Type Ia supernovae are discussed.

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arXiv2006

Turbulent Combustion in Type Ia Supernova Models

We review the astrophysical modeling of type Ia supernova explosions and describe numerical methods to implement numerical simulations of these events. Some results of such simulations are discussed.

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arXiv2006

Off-center ignition in type Ia supernova: I. Initial evolution and implications for delayed detonation

The explosion of a carbon-oxygen white dwarf as a Type Ia supernova is known to be sensitive to the manner in which the burning is ignited. Studies of the pre-supernova evolution suggest asymmetric, off-center ignition, and here we explore its consequences in two- and three-dimensional simulations. Compared with centrally ignited models, one-sided ignitions initially burn less and release less energy. For the distributions of ignition points studied, ignition within two hemispheres typically leads to the unbinding of the white dwarf, while ignition within a small fraction of one hemisphere does not. We also examine the spreading of the blast over the surface of the white dwarf that occurs as the first plumes of burning erupt from the star. In particular, our studies test whether the collision of strong compressional waves can trigger a detonation on the far side of the star as has been suggested by Plewa et al. (2004). The maximum temperature reached in these collisions is sensitive to how much burning and expansion has already gone on, and to the dimensionality of the calculation. Though detonations are sometimes observed in 2D models, none ever happens in the corresponding 3D calculations. Collisions between the expansion fronts of multiple bubbles also seem, in the usual case, unable to ignite a detonation. "Gravitationally confined detonation" is therefore not a robust mechanism for the explosion. Detonation may still be possible in these models however, either following a pulsation or by spontaneous detonation if the turbulent energy is high enough.

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arXiv2006

Theoretical light curves for deflagration models of Type Ia supernova

We present synthetic bolometric and broad-band UBVRI light curves of SNe Ia, for four selected 3-D deflagration models of thermonuclear supernovae. The light curves are computed with the 1-D hydro code STELLA, which models (multi-group time-dependent) non-equilibrium radiative transfer inside SN ejecta. Angle-averaged results from 3-D hydrodynamical explosion simulations with the composition determined in a nucleosynthetic postprocessing step served as the input to the radiative transfer model. The predicted model UBV light curves do agree reasonably well with the observed ones for SNe Ia in the range of low to normal luminosities, although the underlying hydrodynamical explosion models produced only a modest amount of radioactive Ni56 and relatively low kinetic energy in the explosion. The evolution of predicted B and V fluxes in the model with a Ni56 mass of 0.42 M_sun follows the observed decline rate after the maximum very well, although the behavior of fluxes in other filters somewhat deviates from observations, and the bolometric decline rate is a bit slow. Using our models, we check the validity of Arnett's rule and the accuracy of the procedure for extracting the Ni56 mass from the observed light curves. We find that the comparison between theoretical light curves and observations provides a useful tool to validate SN Ia models. The steps necessary to improve the agreement between theory and observations are set out.

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arXiv2006

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 which includes a localised treatment of the energy transfer through the turbulence cascade in combination with semi-statistical closures for the dissipation and non-local transport of turbulence energy. In addition, subgrid scale buoyancy effects are included. In the limit of negligible energy transfer and transport, the dynamical model reduces to the Sharp-Wheeler relation. According to our findings, the Sharp-Wheeler relation is insuffcient to account for the complicated turbulent dynamics of flames in 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, there is a pronounced anisotropy in the vicinity of the flame fronts. The localised subgrid scale model predicts significantly enhanced energy generation and less unburnt carbon and oxygen at low velocities compared to earlier simulations.

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arXiv2005

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.

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arXiv2005

Type Ia supernova diversity in three-dimensional models

The use of type Ia supernovae as distance indicators for cosmology has initiated a search for theoretical arguments supporting the empirical calibration methods applied. To this end, as a first step, a sound understanding of the origin of the observed diversity in type Ia supernova properties is needed. Here we present a first systematic study of effects resulting from changing some physical parameters of three-dimensional deflagration models of thermonuclear supernovae. In our study we vary the progenitor's carbon-to-oxygen ratio and its central density prior to ignition because both properties are not well determined by stellar evolution theory and they may change from supernova to supernova. Next we compute for these explosion models the nucleosynthesis yields in a post-processing step. This, in addition, allows us to study variations in the progenitor's metallicity by means of different $^{22}$Ne mass fractions in the initial composition. We find that the progenitor's carbon-to-oxygen ratio and its central density affect the energy release of the models and thus the expansion velocity of the supernova. Moreover, we find that changing the metallicity and the central density changes the production of radioactive $^{56}$Ni and thus affects the luminosity. In contrast, the carbon-to-oxygen ratio has little effect on the $^{56}$Ni production. Implications of the found variations of the explosion energy and the produced $^{56}$Ni mass for the type Ia supernova diversity are discussed.

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arXiv2004

The distributed burning regime in Type Ia supernova models

The deflagration mode of flame propagation in Type Ia supernova (SN Ia) models requires a correct description of the interaction of the flame with turbulent motions. It is well-known that turbulent combustion proceeds in different regimes. For most parts of the deflagration in SNe Ia the flamelet regime applies. This has been modeled in previous multi-dimensional simulations. However, at late stages of the explosion, the flame will propagate in the so-called distributed regime. We investigate the effects of this regime on SN Ia models in a first and simplified approach and show that the trend of effects seems capable of curing some problems of current pure deflagration models.

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arXiv2004

Full-star Type Ia supernova explosion models

We present full-star simulations of Type Ia supernova explosions on the basis of the standard Chandrasekhar-mass deflagration model. Most simulations so far considered only one spatial octant and assumed mirror symmetry to the other octants. Two full-star models are evolved to homologous expansion and compared with previous single-octant simulations. Therefrom we analyze the effect of abolishing the artificial symmetry constraint on the evolution of the flame surface. It turns out that the development of asymmetries depends on the chosen initial flame configuration. Such asymmetries of the explosion process could possibly contribute to the observed polarization of some Type Ia supernova spectra.

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arXiv2004

Following multi-dimensional Type Ia supernova explosion models to homologous expansion

The last years have witnessed a rapid development of three-dimensional models of Type Ia supernova explosions. Consequently, the next step is to evaluate these models under variation of the initial parameters and to compare them with observations. To calculate synthetic lightcurves and spectra from numerical models, it is mandatory to follow the evolution up to homologous expansion. We report on methods to achieve this in our current implementation of multi-dimensional Type Ia supernova explosion models. The novel scheme is thoroughly tested in two dimensions and a simple example of a three-dimensional simulation is presented. We discuss to what degree the assumption of homologous expansion is justified in these models.

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arXiv2004

Simulations of Turbulent Thermonuclear Burning in Type Ia Supernovae

Type Ia supernovae have recently received considerable attention because it appears that they can be used as "standard candles" to measure cosmic distances out to billions of light years away from us. Observations of type Ia supernovae seem to indicate that we are living in a universe that started to accelerate its expansion when it was about half its present age. These conclusions rest primarily on phenomenological models which, however, lack proper theoretical understanding, mainly because the explosion process, initiated by thermonuclear fusion of carbon and oxygen into heavier elements, is difficult to simulate even on supercomputers. Here, we investigate a new way of modeling turbulent thermonuclear deflagration fronts in white dwarfs undergoing a type Ia supernova explosion. Our approach is based on a level set method which treats the front as a mathematical discontinuity and allows for full coupling between the front geometry and the flow field. New results of the method applied to the problem of type Ia supernovae are obtained. It is shown that in 2-D with high spatial resolution and a physically motivated subgrid scale model for the nuclear flames numerically "converged" results can be obtained, but for most initial conditions the stars do not explode. In contrast, simulations in 3-D, do give the desired explosions and many of their properties, such as the explosion energies, lightcurves and nucleosynthesis products, are in very good agreement with observed type Ia supernovae.

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arXiv2004

On the Stability of Thermonuclear Burning Fronts in Type Ia Supernovae

The propagation of cellularly stabilized thermonuclear flames is investigated by means of numerical simulations. In Type Ia supernova explosions the corresponding burning regime establishes at scales below the Gibson length. The cellular flame stabilization - which is a result of an interplay between the Landau-Darrieus instability and a nonlinear stabilization mechanism - is studied for the case of propagation into quiescent fuel as well as interaction with vortical fuel flows. Our simulations indicate that in thermonuclear supernova explosions stable cellular flames develop around the Gibson scale and that deflagration-to-detonation transition is unlikely to be triggered from flame evolution effects here.

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arXiv2004

The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type Ia supernovae

One of the major challenges for theoretical modeling of Type Ia supernova explosions is to explain the diversity of these events and the empirically established correlation between their peak luminosity and light curve shape. In the framework of the so-called Chandrasekhar mass models, the progenitor's carbon-to-oxygen ratio has been suggested to be a principal source of peak luminosity variations due to a variation in the production of radioactive $^{56}$Ni during the explosion. The underlying idea is that an enhanced carbon mass fraction should result in a more vigorous explosion since here the energy release from nuclear reactions is increased. It was suspected that this would produce a higher amount of $^{56}$Ni in the ejecta. In this letter we describe a mechanism resulting from an interplay between nucleosynthesis and turbulent flame evolution which counteracts such an effect. Based on three-dimensional simulations we argue that it is nearly balanced and only minor differences in the amount of synthesized $^{56}$Ni with varying carbon mass fraction in the progenitor can be expected. Therefore this progenitor parameter is unlikely to account for the observed variations in Type Ia supernova luminosity. We discuss possible effects on the calibration of cosmological measurements.

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arXiv2003

The Cellular Burning Regime in Type Ia Supernova Explosions - II. Flame Propagation into Vortical Fuel

We investigate the interaction of thermonuclear flames in Type Ia supernova explosions with vortical flows by means of numerical simulations. In our study, we focus on small scales, where the flame propagation is no longer dominated by the turbulent cascade originating from large-scale effects. Here, the flame propagation proceeds in the cellular burning regime, resulting from a balance between the Landau-Darrieus instability and its nonlinear stabilization. The interaction of a cellularly stabilized flame front with a vortical fuel flow is explored applying a variety of fuel densities and strengths of the velocity fluctuations. We find that the vortical flow can break up the cellular flame structure if it is sufficiently strong. In this case the flame structure adapts to the imprinted flow field. The transition from the cellularly stabilized front to the flame structure dominated by vortices of the flow proceeds in a smooth way. The implications of the results of our simulations for Type Ia Supernova explosion models are discussed.

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arXiv2003

The Cellular Burning Regime in Type Ia Supernova Explosions - I. Flame Propagation into Quiescent Fuel

We present a numerical investigation of the cellular burning regime in Type Ia supernova explosions. This regime holds at small scales (i.e. below the Gibson scale), which are unresolved in large-scale Type Ia supernova simulations. The fundamental effects that dominate the flame evolution here are the Landau-Darrieus instability and its nonlinear stabilization, leading to a stabilization of the flame in a cellular shape. The flame propagation into quiescent fuel is investigated addressing the dependence of the simulation results on the specific parameters of the numerical setup. Furthermore, we investigate the flame stability at a range of fuel densities. This is directly connected to the questions of active turbulent combustion (a mechanism of flame destabilization and subsequent self-turbulization) and a deflagration-to-detonation transition of the flame. In our simulations we find no substantial destabilization of the flame when propagating into quiescent fuels of densities down to ~10^7 g/cm^3, corroborating fundamental assumptions of large-scale SN Ia explosion models. For these models, however, we suggest an increased lower cutoff for the flame propagation velocity to take the cellular burning regime into account.

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arXiv2003

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

We present a numerical model which allows us to investigate thermonuclear flames in Type Ia supernova explosions. The model is based on a finite-volume explicit hydrodynamics solver employing PPM. Using the level-set technique combined with in-cell reconstruction and flux-splitting schemes we are able to describe the flame in the discontinuity approximation. We apply our implementation to flame propagation in Chandrasekhar-mass Type Ia supernova models. In particular we concentrate on intermediate scales between the flame width and the Gibson-scale, where the burning front is subject to the Landau-Darrieus instability. We are able to reproduce the theoretical prediction on the growth rates of perturbations in the linear regime and observe the stabilization of the flame in a cellular shape. The increase of the mean burning velocity due to the enlarged flame surface is measured. Results of our simulation are in agreement with semianalytical studies.

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