David A. Chamulak

The Reduction of the Electron Abundance during the Pre-explosion Simmering in White Dwarf Supernovae

Prior to the explosion of a carbon-oxygen white dwarf in a Type Ia supernova there is a long “simmering,” during which the 12 C + 12 C reaction gradually heats the white dwarf on a long (∼ 10 3 yr) timescale. Piro & Bildsten showed that weak reactions during this simmering set a maximum electron abundance Ye at the time of the explosion. We investigate the nuclear reactions duri ng this simmering with a series of self-heating, at constant pressure, reaction network calculations. Unlike in AGB stars, proton captures onto 22 Ne and heavier trace nuclei do not play a significant role. The 12 C abundance is suffi ciently high that the neutrons preferentially capture onto 12 C, rather than iron group nuclei. As an aid to hydrodynamical simulations of the simmering phase, we present fits to the rates of heating, electron captu re, change in mean atomic mass, and consumption of 12 C in terms of the screened thermally averaged cross section f or 12 C + 12 C. Our evaluation of the net heating rate includes contributions from electron capture s into the 3.68 MeV excited state of 13 C. This results in a slightly larger energy release, per 12 C consumed, than that found by Piro & Bildsten, but less than that released for a burn to only 20 Ne and 23 Na. We compare our one-zone results to more accurate integrations over the white dwarf structure to estimate the amount of 12 C that must be consumed to raise the white dwarf temperature, and hence to determine the net reduction of Ye during simmering. Subject headings:nuclear reactions, nucleosynthesis, abundances — supernovae: general — white dwarfs — galaxies: evolution

ASYMMETRY AND THE NUCLEOSYNTHETIC SIGNATURE OF NEARLY EDGE-LIT DETONATION IN WHITE DWARF CORES

Most of the leading explosion scenarios for Type Ia supernovae involve the nuclear incineration of a white dwarf star through a detonation wave. Several scenarios have been proposed as to how this detonation may actually occur, but the exact mechanism and environment in which it takes place remain unknown. We explore the effects of an off-center initiated detonation on the spatial distribution of the nucleosynthetic yield products in a toy model?a pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point near edge-lit detonation results in asymmetries in the density and thermal profiles, notably the expansion timescale, throughout the supernova ejecta. We demonstrate that this asymmetry of the thermodynamic trajectories should be common to off-center detonations where a small amount of the star is burned prior to detonation. The sensitivity of the yields on the expansion timescale results in an asymmetric distribution of the elements synthesized as reaction products. We tabulate the shift in the center of mass of the various elements produced in our model supernova and find an odd-even pattern for elements past silicon. Our calculations show that off-center single-point detonations in carbon-oxygen white dwarfs are marked by significant composition asymmetries in their remnants which bear potentially observable signatures in both velocity and coordinate space, including an elemental nickel mass fraction that varies by a factor of 2-3 from one side of the remnant to the other.

On silicon group elements ejected by supernovae type IA

There is evidence that the peak brightness of a Type Ia supernova is affected by the electron fraction Y-e at the time of the explosion. The electron fraction is set by the aboriginal composition of the white dwarf and the reactions that occur during the pre-explosive convective burning. To date, determining the makeup of the white dwarf progenitor has relied on indirect proxies, such as the average metallicity of the host stellar population. In this paper, we present analytical calculations supporting the idea that the electron fraction of the progenitor systematically influences the nucleosynthesis of silicon group ejecta in Type Ia supernovae. In particular, we suggest the abundances generated in quasi-nuclear statistical equilibrium are preserved during the subsequent freeze-out. This allows potential recovery of Y-e at explosion from the abundances recovered from an observed spectra. We show that measurement of Si-28, S-32, Ca-40, and Fe-54 abundances can be used to construct Y-e in the silicon-rich regions of the supernovae. If these four abundances are determined exactly, they are sufficient to recover Y-e to 6%. This is because these isotopes dominate the composition of silicon-rich material and iron-rich material in quasi-nuclear statistical equilibrium. Analytical analysis shows the Si-28 abundance is insensitive to Y-e, the S-32 abundance has a nearly linear trend with Y-e, and the Ca-40 abundance has a nearly quadratic trend with Y-e. We verify these trends with post-processing of one-dimensional models and show that these trends are reflected in the models synthetic spectra.

Turbulence-flame interaction on the early evolution of flames in type Ia supernovae

Arizona State University Type Ia supernovae are bright stellar explosions thought to occur when a runaway thermonuclear reaction incinerates a compact star known as white dwarf (WD). In many models, the explosion begins with a flame born in the turbulent environment near the center of the white dwarf. The effect of turbulence on the evolution of the nascent flame is incompletely understood and is the subject of active research. The range of length scales from the full star (∼ 108 cm) to the laminar flame width (∼ 10−5 cm) prevents full-star simulations from resolving the turbulence-flame interaction (TFI) directly. In the single-degenerate paradigm of Type Ia supernovae (SNe Ia), the WD experiences ∼ 1000 year period of convection as the temperature rises to burn carbon. When the nuclear burning timescale exceeds the turnover time for convective eddies, a flame is born in the center of a vigorous convection field (vrms ∼ 400 km/s) extending out to enclose ∼ 70% of the WD’s mass [1]. We present preliminary results from a physically-motivated TFI model inspired by Colin et al. (2000) [2] that utilizes a local, instantaneous measure of the turbulence to enhance the flame speed due to under-resolved TFI. We explore various implementation choices in the TFI model and compare results to previous work. We present two simulations of the early flame evolution in a supernova. One incorporates a TFI model with particular implementation choices, while the other relies only on indirect buoyancy effects [3]. 11th Symposium on Nuclei in the Cosmos, NIC XI July 19-23, 2010 Heidelberg, Germany

Laminar flame acceleration by neon enrichment in white dwarf supernovae

We explore how the laminar flame speed of degenerate C/O thermonuclear burning during a type Ia supernova depends on the composition of the white dwarf. Type Ia supernovae are currently the premier standard candle for measuring distances to redshift . 1.6. The currently favored scenario for this supernovae is the thermonuclear incineration of a C/O white dwarf. Recent observations suggest that there may be more than one population of progenitor, and it has been suggested the peak luminosity may depend on the composition of the progenitor white dwarf. Of particular interest is22Ne, which is formed from CNO elements during core He burning of the progenitor star and therefore reflects the metallicity of the progenitor. We find that the laminar flame speed of a C/O mixture increases linearly with the abundance of 22Ne when the abundance of 22Ne is small. The faster and narrower laminar flame enlarges the lengthscale at which turbulent eddies can disrupt the burn. As a result, the addition of 22Ne might lower the density at which a transition to distributed burning occurs.