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.Comment: 13 pages, 10 figures (resolution of some figures reduced to comply with astro-ph file size restriction); submitted to the Astrophysical Journal on 8/3/200

Modeling the Diversity of Type Ia Supernova Explosions

Type Ia supernovae (SNe Ia) are a prime tool in observational cosmology. A relation between their peak luminosities and the shapes of their light curves allows to infer their intrinsic luminosities and to use them as distance indicators. This relation has been established empirically. However, a theoretical understanding is necessary in order to get a handle on the systematics in SN Ia cosmology. Here, a model reproducing the observed diversity of normal SNe Ia is presented. The challenge in the numerical implementation arises from the vast range of scales involved in the physical mechanism. Simulating the supernova on scales of the exploding white dwarf requires specific models of the microphysics involved in the thermonuclear combustion process. Such techniques are discussed and results of simulations are presented.Comment: 6 pages, ASTRONUM-2009 "Numerical Modeling of Space Plasma Flows", Chamonix, France, July 2009, to appear in ASP Conf. Pro

A Common Explosion Mechanism for Type Ia Supernovae

Type Ia supernovae, the thermonuclear explosions of white dwarf stars composed of carbon and oxygen, were instrumental as distance indicators in establishing the acceleration of the universe's expansion. However, the physics of the explosion are debated. Here we report a systematic spectral analysis of a large sample of well observed type Ia supernovae. Mapping the velocity distribution of the main products of nuclear burning, we constrain theoretical scenarios. We find that all supernovae have low-velocity cores of stable iron-group elements. Outside this core, nickel-56 dominates the supernova ejecta. The outer extent of the iron-group material depends on the amount of nickel-56 and coincides with the inner extent of silicon, the principal product of incomplete burning. The outer extent of the bulk of silicon is similar in all SNe, having an expansion velocity of ~11000 km/s and corresponding to a mass of slightly over one solar mass. This indicates that all the supernovae considered here burned similar masses, and suggests that their progenitors had the same mass. Synthetic light curve parameters and three-dimensional explosion simulations support this interpretation. A single explosion scenario, possibly a delayed detonation, may thus explain most type Ia supernovae.Comment: 8 pages, 2 figure

Oxygen emission in remnants of thermonuclear supernovae as a probe for their progenitor system

Recent progress in numerical simulations of thermonuclear supernova explosions brings up a unique opportunity in studying the progenitors of Type Ia supernovae. Coupling state-of-the-art explosion models with detailed hydrodynamical simulations of the supernova remnant evolution and the most up-to-date atomic data for X-ray emission calculations makes it possible to create realistic synthetic X-ray spectra for the supernova remnant phase. Comparing such spectra with high quality observations of supernova remnants could allow to constrain the explosion mechanism and the progenitor of the supernova. The present study focuses in particular on the oxygen emission line properties in young supernova remnants, since different explosion scenarios predict a different amount and distribution of this element. Analysis of the soft X-ray spectra from supernova remnants in the Large Magellanic Cloud and confrontation with remnant models for different explosion scenarios suggests that SNR 0509-67.5 could originate from a delayed detonation explosion and SNR 0519-69.0 from an oxygen-rich merger.Comment: 8 pages, 4 figures, MNRAS accepte

Early light curves for Type Ia supernova explosion models

Upcoming high-cadence transient survey programmes will produce a wealth of observational data for Type Ia supernovae. These data sets will contain numerous events detected very early in their evolution, shortly after explosion. Here, we present synthetic light curves, calculated with the radiation hydrodynamical approach Stella for a number of different explosion models, specifically focusing on these first few days after explosion. We show that overall the early light curve evolution is similar for most of the investigated models. Characteristic imprints are induced by radioactive material located close to the surface. However, these are very similar to the signatures expected from ejecta-CSM or ejecta-companion interaction. Apart from the pure deflagration explosion models, none of our synthetic light curves exhibit the commonly assumed power-law rise. We demonstrate that this can lead to substantial errors in the determination of the time of explosion. In summary, we illustrate with our calculations that even with very early data an identification of specific explosion scenarios is challenging, if only photometric observations are available.Comment: 15 pages, 14 figures, 3 tables, accepted for publication in MNRA

Spectral modeling of type II supernovae. I. Dilution factors

We present substantial extensions to the Monte Carlo radiative transfer code TARDIS to perform spectral synthesis for type II supernovae. By incorporating a non-LTE ionization and excitation treatment for hydrogen, a full account of free-free and bound-free processes, a self-consistent determination of the thermal state and by improving the handling of relativistic effects, the improved code version includes the necessary physics to perform spectral synthesis for type II supernovae to high precision as required for the reliable inference of supernova properties. We demonstrate the capabilities of the extended version of TARDIS by calculating synthetic spectra for the prototypical type II supernova SN1999em and by deriving a new and independent set of dilution factors for the expanding photosphere method. We have investigated in detail the dependence of the dilution factors on photospheric properties and, for the first time, on changes in metallicity. We also compare our results with two previously published sets of dilution factors by Eastman et al. (1996) and by Dessart & Hillier (2005), and discuss the potential sources of the discrepancies between studies.Comment: 16 pages, 12 figures, 2 tables, accepted for publication in A&

r-Process in Prompt Supernova Explosions Revisited

We reanalyze $r$-process nucleosynthesis in the neutron-rich ejecta from a prompt supernova explosion of a low-mass (11 M$_\odot$) progenitor. A pompt explosion is not yet ruled out as a possibility for low-mass supernova progenitors. Moreover, there is mounting evidence that a new $r$-process site may be required. Hence, we assume that a prompt explosion can occur and make a study of r-process nucleosynthesis in the supernova ejecta. To achieve a prompt explosion we have performed a general relativistic hydrodynamic simulation of adiabatic collapse and bounce using a relativistic nuclear-matter equation of state. The electron fraction $Y_e$ during the collapse was fixed at the initial-model value. The size of the inner collapsing core was then large enough to enable a prompt explosion to occur. Adopting the calculated trajectories of promptly ejected material, we explicitly computed the burst of neutronization due to electron captures on free protons in the photodissociated ejecta after the passage of the shock. The thermal and compositional evolution of the resulting neutron-rich ejecta originating from near the surface of proto-neutron star was obtained. These were used in nuclear reaction network calculations to evaluate the products of $r$-process nucleosynthesis. We find that, unlike earlier studies, the amount of $r$-process material ejected per supernova is quite consistent with observed galactic $r$-process abundances. Furthermore, the computed $r$-process abundances are in good agreement with Solar abundances of $r$-process elements for A$> 100$. This suggests that prompt supernovae are still a viable $r$-process site. Such events may be responsible for the abundances of the heaviest $r$-process nuclei.Comment: 13 pages, 8 figures, ApJ in press. Minor revisions as per referee's suggestion

Type Ia Supernovae and Accretion Induced Collapse

Using the population synthesis binary evolution code StarTrack, we present theoretical rates and delay times of Type Ia supernovae arising from various formation channels. These channels include binaries in which the exploding white dwarf reaches the Chandrasekhar mass limit (DDS, SDS, and helium-rich donor scenario) as well as the sub-Chandrasekhar mass scenario, in which a white dwarf accretes from a helium-rich companion and explodes as a SN Ia before reaching the Chandrasekhar mass limit. We find that using a common envelope parameterization employing energy balance with alpha=1 and lambda=1, the supernova rates per unit mass (born in stars) of sub-Chandrasekhar mass SNe Ia exceed those of all other progenitor channels at epochs t=0.7 - 4 Gyr for a burst of star formation at t=0. Additionally, the delay time distribution of the sub-Chandrasekhar model can be divided in to two distinct evolutionary channels: the `prompt' helium-star channel with delay times < 500 Myr, and the `delayed' double white dwarf channel with delay times > 800 Myr spanning up to a Hubble time. These findings are in agreement with recent observationally-derived delay time distributions which predict that a large number of SNe Ia have delay times < 1 Gyr, with a significant fraction having delay times < 500 Myr. We find that the DDS channel is also able to account for the observed rates of SNe Ia. However, detailed simulations of white dwarf mergers have shown that most of these mergers will not lead to SNe Ia but rather to the formation of a neutron star via accretion-induced collapse. If this is true, our standard population synthesis model predicts that the only progenitor channel which can account for the rates of SNe Ia is the sub-Chandrasekhar mass scenario, and none of the other progenitors considered can fully account for the observed rates.Comment: 6 pages, 1 figure, 1 table, to appear in proceedings for "Binary Star Evolution: Mass Loss, Accretion and Mergers

Applying the expanding photosphere and standardized candle methods to Type II-Plateau supernovae at cosmologically significant redshifts: the distance to SN 2013eq

Based on optical imaging and spectroscopy of the Type II-Plateau SN 2013eq, we present a comparative study of commonly used distance determination methods based on Type II supernovae. The occurrence of SN 2013eq in the Hubble flow (z = 0.041 +/- 0.001) prompted us to investigate the implications of the difference between "angular" and "luminosity" distances within the framework of the expanding photosphere method (EPM) that relies upon a relation between flux and angular size to yield a distance. Following a re-derivation of the basic equations of the EPM for SNe at non-negligible redshifts, we conclude that the EPM results in an angular distance. The observed flux should be converted into the SN rest frame and the angular size, theta, has to be corrected by a factor of (1+z)^2. Alternatively, the EPM angular distance can be converted to a luminosity distance by implementing a modification of the angular size. For SN 2013eq, we find EPM luminosity distances of D_L = 151 +/- 18 Mpc and D_L = 164 +/- 20 Mpc by making use of different sets of dilution factors taken from the literature. Application of the standardized candle method for Type II-P SNe results in an independent luminosity distance estimate (D_L = 168 +/- 16 Mpc) that is consistent with the EPM estimate.Comment: 12 pages, 4 figures, accepted by A&