Synthetic Spectra for Type Ia Supernovae at Early Epochs

We present the current status of our construction of synthetic spectra for type Ia supernovae. These properly take into account the effects of NLTE and an adequate representation of line blocking and blanketing. The models are based on a sophisticated atomic database. We show that the synthetic spectrum reproduces the observed spectrum of 'normal' SN-Ia near maximum light from the UV to the near-IR. However, further improvements are necessary before truly quantitative analyses of observed SN-Ia spectra can be performed. In particular, the inner boundary condition has to be fundamentally modified. This is due to the dominance of electron scattering over true absorption processes coupled with the flat density structure in these objectsComment: To appear in "Proceedings of the IAU Colloquium 192 - Supernovae (10 Years of SN1993J)", eds. J.M. Marcaide and K.W. Weile

The Interaction of High-Speed Turbulence with Flames: Global Properties and Internal Flame Structure

We study the dynamics and properties of a turbulent flame, formed in the presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type turbulence in an unconfined system. Direct numerical simulations are performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion model represents a stoichiometric H2-air mixture. The system being modeled represents turbulent combustion with the Damkohler number Da = 0.05 and with the turbulent velocity at the energy injection scale 30 times larger than the laminar flame speed. The simulations show that flame interaction with high-speed turbulence forms a steadily propagating turbulent flame with a flame brush width approximately twice the energy injection scale and a speed four times the laminar flame speed. A method for reconstructing the internal flame structure is described and used to show that the turbulent flame consists of tightly folded flamelets. The reaction zone structure of these is virtually identical to that of the planar laminar flame, while the preheat zone is broadened by approximately a factor of two. Consequently, the system evolution represents turbulent combustion in the thin-reaction zone regime. The turbulent cascade fails to penetrate the internal flame structure, and thus the action of small-scale turbulence is suppressed throughout most of the flame. Finally, our results suggest that for stoichiometric H2-air mixtures, any substantial flame broadening by the action of turbulence cannot be expected in all subsonic regimes.Comment: 30 pages, 9 figures; published in Combustion and Flam

Explosion models for thermonuclear supernovae resulting from different ignition conditions

We have explored in three dimensions the fate of a massive white dwarf as a function of different initial locations of carbon ignition, with the aid of a SPH code. The calculated models cover a variety of possibilities ranging from the simultaneous ignition of the central volume of the star to the off-center ignition in multiple scattered spots. In the former case, there are discussed the possibility of a transition to a detonation when the mean density of the nuclear flame decreases below 2x10**7 g cm**-3, and its consequences. In the last case, the dependence of the results on the number of initial igniting spots and the chance of some of these models to evolve to the pulsating delayed detonation scenario are also outlined.Comment: 5 pages, 1 figure, proceedings of IAU Colloquium 192, 'Supernovae (10 years of SN1993J)', 22-26 April 2003, Valencia, Spai

Three-dimensional modeling of Type Ia supernovae - The power of late time spectra

Late time synthetic spectra of Type Ia supernovae, based on three-dimensional deflagration models, are presented. We mainly focus on one model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous phase of the explosion. Other models with different ignition conditions and different resolution are also briefly discussed. The synthetic spectra are compared to observed late time spectra. We find that while the model spectra after 300 to 500 days show a good agreement with the observed Fe II-III features, they also show too strong O I and C I lines compared to the observed late time spectra. The oxygen and carbon emission originates from the low-velocity unburned material in the central regions of these models. To get agreement between the models and observations we find that only a small mass of unburned material may be left in the center after the explosion. This may be a problem for pure deflagration models, although improved initial conditions, as well as higher resolution decrease the discrepancy. The relative intensity from the different ionization stages of iron is sensitive to the density of the emitting iron-rich material. We find that clumping, with the presence of low density regions, is needed to reproduce the observed iron emission, especially in the range between 4000 and 6000 AA. Both temperature and ionization depend sensitively on density, abundances and radioactive content. This work therefore illustrates the importance of including the inhomogeneous nature of realistic three-dimensional explosion models. We briefly discuss the implications of the spectral modeling for the nature of the explosion.Comment: 20 pages, 9 figures, resolution of Fig 1 is reduced to meet astro-ph file size restriction, submitted to A&

Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs

In the "double-detonation sub-Chandrasekhar" model for type Ia supernovae, a carbon-oxygen (C + O) white dwarf accumulates sufficient amounts of helium such that a detonation ignites in that layer before the Chandrasekhar mass is reached. This detonation is thought to trigger a secondary detonation in the C + O core. By means of one- and two-dimensional hydrodynamic simulations, we investigate the robustness of this explosion mechanism for generic 1-M_sun models and analyze its observable predictions. Also a resolution dependence in numerical simulations is analyzed. The propagation of thermonuclear detonation fronts, both in helium and in the carbon-oxygen mixture, is computed by means of both a level-set function and a simplified description for nuclear reactions. The decision whether a secondary detonation is triggered in the white dwarf's core or not is made based on criteria given in the literature. In a parameter study involving different initial flame geometries for He-shell masses of 0.2 and 0.1 M_sun, we find that a secondary detonation ignition is a very robust process. Converging shock waves originating from the detonation in the He shell generate the conditions for a detonation near the center of the white dwarf in most of the cases considered. Finally, we follow the complete evolution of three selected models with 0.2 M_sun of He through the C/O-detonation phase and obtain nickel-masses of about 0.40 to 0.45 M_sun. Although we have not done a complete scan of the possible parameter space, our results show that sub-Chandrasekhar models are not good candidates for normal or sub-luminous type Ia supernovae. The chemical composition of the ejecta features significant amounts of nickel in the outer layers at high expansion velocities, which is inconsistent with near-maximum spectra. (abbreviated)Comment: 11 pages, 10 figures, PDFLaTeX, accepted for publication in A&

C+O detonations in thermonuclear supernovae: Interaction with previously burned material

In the context of explosion models for Type Ia Supernovae, we present one- and two-dimensional simulations of fully resolved detonation fronts in degenerate C+O White Dwarf matter including clumps of previously burned material. The ability of detonations to survive the passage through sheets of nuclear ashes is tested as a function of the width and composition of the ash region. We show that detonation fronts are quenched by microscopically thin obstacles with little sensitivity to the exact ash composition. Front-tracking models for detonations in macroscopic explosion simulations need to include this effect in order to predict the amount of unburned material in delayed detonation scenarios.Comment: 6 pages, 9 figures, uses isotope.sty, accepted for publication in A&

Is there a hidden hole in Type Ia supernova remnants?

In this paper we report on the bulk features of the hole carved by the companion star in the material ejected during a Type Ia supernova explosion. In particular we are interested in the long term evolution of the hole as well as in its fingerprint in the geometry of the supernova remnant after several centuries of evolution, which is a hot topic in current Type Iasupernovae studies. We use an axisymmetric smoothed particle hydrodynamics code to characterize the geometric properties of the supernova remnant resulting from the interaction of this ejected material with the ambient medium. Our aim is to use supernova remnant observations to constrain the single degenerate scenario for Type Ia supernova progenitors. Our simulations show that the hole will remain open during centuries, although its partial or total closure at later times due to hydrodynamic instabilities is not excluded. Close to the edge of the hole, the Rayleigh-Taylor instability grows faster, leading to plumes that approach the edge of the forward shock. We also discuss other geometrical properties of the simulations, like the evolution of the contact discontinuity.Comment: 48 pages, 17 figures; Accepted for publication in Ap

Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles

Nucleosynthetic yield predictions for multi-dimensional simulations of thermonuclear supernovae generally rely on the tracer particle method to obtain isotopic information of the ejected material for a given supernova simulation. We investigate how many tracer particles are required to determine converged integrated total nucleosynthetic yields. For this purpose, we conduct a resolution study in the number of tracer particles for different hydrodynamical explosion models at fixed spatial resolution. We perform hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry for both pure deflagration and delayed detonation Type Ia supernova explosions. Within a given explosion model, we vary the number of tracer particles to determine the minimum needed for the method to give a robust prediction of the integrated yields of the most abundant nuclides. For the first time, we relax the usual assumption of constant tracer particle mass and introduce a radially vary- ing distribution of tracer particle masses. We find that the nucleosynthetic yields of the most abundant species (mass fraction > 10E-5) are reasonably well predicted for a tracer number as small as 32 per axis and direction - more or less independent of the explosion model. We conclude that the number of tracer particles that were used in extant published works appear to have been sufficient as far as integrated yields are concerned for the most copiously produced nuclides. Additionally we find that a suitably chosen tracer mass distribution can improve convergence for nuclei produced in the outer layer of the supernova where the constant tracer mass prescription suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA

New evidence for strong nonthermal effects in Tycho's supernova remnant

For the case of Tycho's supernova remnant (SNR) we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is so close to the shock radius that it can only be explained by efficient CR acceleration which in turn makes the medium more compressible. Together with the recently determined new value $E_{sn}=1.2\times 10^{51}$ erg of the SN explosion energy this also confirms our previous conclusion that a TeV gamma-ray flux of $(2-5)\times 10^{-13}$ erg/(cm$^2$s) is to be expected from Tycho's SNR. Chandra measurements and the HEGRA upper limit of the TeV gamma-ray flux together limit the source distance $d$ to $3.3\leq d\leq 4$ kpc.Comment: 5 pages, 4 figures. Accepted for publication in Astrophysics and Space Science, Proc. of "The Multi-Messenger Approach to High-Energy Gamma-ray Sources (Third Workshop on the Nature of Unidentified High-Energy Sources)", Barcelona, July 4-7, 200

Low Mach Number Modeling of Type Ia Supernovae. IV. White Dwarf Convection

We present the first three-dimensional, full-star simulations of convection in a white dwarf preceding a Type Ia supernova, specifically the last few hours before ignition. For these long-time calculations we use our low Mach number hydrodynamics code, MAESTRO, which we have further developed to treat spherical stars centered in a three-dimensional Cartesian geometry. The main change required is a procedure to map the one-dimensional radial base state to and from the Cartesian grid. Our models recover the dipole structure of the flow seen in previous calculations, but our long-time integration shows that the orientation of the dipole changes with time. Furthermore, we show the development of gravity waves in the outer, stable portion of the star. Finally, we evolve several calculations to the point of ignition and discuss the range of ignition radii.Comment: 42 pages, some figures degraded to conserve space. Accepted to The Astrophysical Journal (http://journals.iop.org/