arXiv subjects

M. Reinecke

Publications and source records attributed to M. Reinecke.

70 records · Page 4Linked to original sources

arXiv2006

Making sky maps from Planck data

We compare the performance of multiple codes written by different groups for making polarized maps from Planck-sized, all-sky cosmic microwave background (CMB) data. Three of the codes are based on a destriping algorithm; the other three are implementations of an optimal maximum-likelihood algorithm. Time-ordered data (TOD) were simulated using the Planck Level-S simulation pipeline. Several cases of temperature-only data were run to test that the codes could handle large datasets, and to explore effects such as the precision of the pointing data. Based on these preliminary results, TOD were generated for a set of four 217 GHz detectors (the minimum number required to produce I, Q, and U maps) under two different scanning strategies, with and without noise. Following correction of various problems revealed by the early simulation, all codes were able to handle the large data volume that Planck will produce. Differences in maps produced are small but noticeable; differences in computing resources are large.

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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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arXiv2005

Metallicity effect in multi-dimensional SNIa nucleosynthesis

We investigate the metallicity effect (measured by the original 22Ne content) on the detailed nucleosynthetic yields for 3D hydrodynamical simulations of the thermonuclear burning phase in SNe Ia. Calculations are based on post-processes of the ejecta, using passively advected tracer particles, as explained in details by Travaglio et al.(2004). The nuclear reaction network employed in computing the explosive nucleosynthesis contains 383 nuclear species. For this work we use the high resolution multi-point ignition (bubbles) model b30_3d_768 (Travaglio et al.2004 for the solar metallicity case), and we cover a metallicity range between 0.1xZ_sun up to 3xZ_sun. We find a linear dependence of the 56Ni mass ejected on the progenitor's metallicity, with a variation in the 56Ni mass of ~25% in the metallicity range explored. Moreover, the largest variation in 56Ni occurs at metallicity greater than solar. Almost no variations are shown in the unburned material 12C and 16O. The largest metallicity effect is seen in the alpha-elements. Finally, implications for the observed scatter in the peak luminosities of SNe Ia are also discussed.

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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

HEALPix -- a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere

HEALPix -- the Hierarchical Equal Area iso-Latitude Pixelization -- is a versatile data structure with an associated library of computational algorithms and visualization software that supports fast scientific applications executable directly on very large volumes of astronomical data and large area surveys in the form of discretized spherical maps. Originally developed to address the data processing and analysis needs of the present generation of cosmic microwave background (CMB) experiments (e.g. BOOMERanG, WMAP), HEALPix can be expanded to meet many of the profound challenges that will arise in confrontation with the observational output of future missions and experiments, including e.g. Planck, Herschel, SAFIR, and the Beyond Einstein CMB polarization probe. In this paper we consider the requirements and constraints to be met in order to implement a sufficient framework for the efficient discretization and fast analysis/synthesis of functions defined on the sphere, and summarise how they are satisfied by HEALPix.

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arXiv2004

Nucleosynthesis in multi-dimensional SNIa explosions

We present the results of nucleosynthesis calculations based on multidimensional (2D and 3D) hydrodynamical simulations of the thermonuclear burning phase in SNIa. The detailed nucleosynthetic yields of our explosion models are calculated by post-processing the ejecta, using passively advected tracer particles. The nuclear reaction network employed in computing the explosive nucleosynthesis contains 383 nuclear species. We analyzed two different choices of ignition conditions (centrally ignited, in which the spherical initial flame geometry is perturbated with toroidal rings, and bubbles, in which multi-point ignition conditions are simulated). We show that unburned C and O varies typically from ~40% to ~50% of the total ejected material.The main differences between all our models and standard 1D computations are, besides the higher mass fraction of unburned C and O, the C/O ratio (in our case is typically a factor of 2.5 higher than in 1D computations), and somewhat lower abundances of certain intermediate mass nuclei such as S, Cl, Ar, K, and Ca, and of 56Ni. Because explosive C and O burning may produce the iron-group elements and their isotopes in rather different proportions one can get different 56Ni-fractions (and thus supernova luminosities) without changing the kinetic energy of the explosion. Finally, we show that we need the high resolution multi-point ignition (bubbles) model to burn most of the material in the center (demonstrating that high resolution coupled with a large number of ignition spots is crucial to get rid of unburned material in a pure deflagration SNIa model).

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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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arXiv2002

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.

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arXiv2002

Models of Type Ia Supernova Explosions

Type Ia supernovae have become an indispensable tool for studying the expansion history of the universe, yet our understanding of the explosion mechanism is still incomplete. We describe the variety of discussed scenarios, sketch the most relevant physics, and report recent advances in multidimensional simulations of Chandrasekhar mass white dwarf explosions.

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arXiv2001

On the Explosion Mechanism of SNe Type Ia

In this article we discuss the first simulations of two- and three-dimensional Type Ia supernovae with an improved hydrodynamics code. After describing the various enhancements, the obtained results are compared to those of earlier code versions, observational data and the findings of other researchers in this field.

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arXiv2001

Refined numerical models for multidimensional Type Ia supernova simulations

Following up on earlier work on this topic (Reinecke et al. 1999, A&A 347, pp. 724 and 739), 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.

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arXiv2000

Supernovae as stellar objects

Type Ia supernovae (SN Ia) are generally believed to be the result of the thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs, mainly because such thermonuclear explosions can account for the right amount of nickel, which is needed to explain the light curves and the late-time spectra, and the abundances of intermediate-mass nuclei which dominate the spectra near maximum light. Because of their enormous brightness and apparent homogeneity SN Ia have become an important tool to measure cosmological parameters. In this article the present understanding of the physics of thermonuclear explosions is reviewed. In particular, we focus our attention on subsonic ("deflagration") fronts, i.e. we investigate fronts propagating by heat diffusion and convection rather than by compression. Models based upon this mode of nuclear burning have been applied very successfully to the SN Ia problem, and are able to reproduce many of their observed features remarkably well. However, the models also indicate that SN Ia may differ considerably from each other, which is of importance if they are to be used as standard candles.

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arXiv2000

Type Ia Supernova Explosion Models: Homogeneity versus Diversity

Type Ia supernovae (SN Ia) are generally believed to be the result of the thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs, mainly because such thermonuclear explosions can account for the right amount of Ni-56, which is needed to explain the light curves and the late-time spectra, and the abundances of intermediate-mass nuclei which dominate the spectra near maximum light. Because of their enormous brightness and apparent homogeneity SN Ia have become an important tool to measure cosmological parameters. In this article the present understanding of the physics of thermonuclear explosions is reviewed. In particular, we focus our attention on subsonic (``deflagration'') fronts, i.e. we investigate fronts propagating by heat diffusion and convection rather than by compression. Models based upon this mode of nuclear burning have been applied very successfully to the SN Ia problem, and are able to reproduce many of their observed features remarkably well. However, the models also indicate that SN Ia may differ considerably from each other, which is of importance if they are to be used as standard candles.

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arXiv2000

Thermonuclear Explosions of Chandrasekhar-Mass White Dwarfs

We present a new way of modeling turbulent thermonuclear deflagration fronts in Chandrasekhar-mass white dwarfs, consisting of carbon and oxygen, undergoing a type Ia supernova explosion. Our approach is a front capturing/tracking hybrid scheme, 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. First results of the method applied to the problem of type Ia supernovae are discussed. It will be shown that even in 2-D and even with a physically motivated sub-grid model numerically ``converged'' results are difficult to obtain.

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arXiv1999

Thermonuclear explosions of Chandrasekhar-mass C+O white dwarfs

First results of simulations are presented which compute the dynamical evolution of a Chandrasekhar-mass white dwarf, consisting of equal amounts of carbon and oxygen, from the onset of violent thermonuclear burning, by means of a new two-dimensional numerical code. Since in the interior of such a massive white dwarf nuclear burning progresses on microscopic scales as a sharp discontinuity, a so-called flamelet, which cannot be resolved by any numerical scheme, and since on macroscopic scales the burning front propagates due to turbulence, we make an attempt to model both effects explicitly in the framework of a finite-volume hydrodynamics code. Turbulence is included by a sub-grid model, following the spirit of large eddy simulations, and the well-localized burning front is treated by means of a level set, which allows us to compute the geometrical structure of the front more accurately than with previous methods. The only free parameters of our simulations are the location and the amount of nuclear fuel that is ignited as an initial perturbation. We find that models in which explosive carbon burning is ignited at the center remain bound by the time the front reaches low densities, where we stopped the computations because our description of combustion is no longer applicable. In contrast, off-center ignition models give rise to explosions which, however, are still too weak for typical Type Ia supernovae. Possible reasons for this rather disappointing result are discussed.

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arXiv1999

A new model for deflagration fronts in reactive fluids

We present a new way of modeling deflagration fronts in reactive fluids, the main emphasis being on 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 full coupling between the front geometry and the flow field. With only minor modifications, this method can also be applied to describe contact discontinuities. Two different implementations are described and their physically correct behaviour for simple testcases is shown. First results of the method applied to the concrete problems of Type Ia supernovae and chemical hydrogen combustion are briefly discussed; a more extensive analysis of our astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report 1122b).

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