Browsing by Author "Calder, Alan C."
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Item Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Central Density(2012-10-01) Krueger, Brendan K.; Jackson, Aaron P.; Calder, Alan C.; Townsley, Dean M.; Brown, Edward F.; Timmes, Francis X.; University of Alabama TuscaloosaWe present a study exploring a systematic effect on the brightness of Type Ia supernovae using numerical models that assume the single-degenerate paradigm. Our investigation varied the central density of the progenitor white dwarf at flame ignition, and considered its impact on the explosion yield, particularly the production and distribution of radioactive 56Ni, which powers the light curve. We performed a suite of two-dimensional simulations with randomized initial conditions, allowing us to characterize the statistical trends that we present. The simulations indicate that the production of Fe-group material is statistically independent of progenitor central density, but the mass of stable Fe-group isotopes is tightly correlated with central density, with a decrease in the production of 56Ni at higher central densities. These results imply that progenitors with higher central densities produce dimmer events. We provide details of the post-explosion distribution of 56Ni in the models, including the lack of a consistent centrally located deficit of 56Ni, which may be compared to observed remnants. By performing a self-consistent extrapolation of our model yields and considering the main-sequence lifetime of the progenitor star and the elapsed time between the formation of the white dwarf and the onset of accretion, we develop a brightness–age relation that improves our prediction of the expected trend for single degenerates and we compare this relation with observations.Item Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Deflagration to Detonation Density(2011) Calder, Alan C.; Jackson, Aaron P.; Krueger, Brendan K.; Townsley, Dean M.; Chamulak, David A.; Brown, Edward F.; Timmes, F.X.; University of Alabama TuscaloosaA widely accepted setting for type Ia supernovae (SNeIa) is a thermonuclear runaway occurring in a C/O white dwarf (WD) that gained mass from a companion. The peak brightness is determined by the mass of radioactive 56Ni synthesized that powers the light curve. Models that best agree with observations begin with a subsonic deflagration that transitions to a supersonic detonation that rapidly incinerates the star. The condition under which the deflagration-to-detonation transition (DDT) occurs is largely uncertain and remains essentially a free parameter. We parameterize the DDT in terms of the local density because the characteristics of the burning wave depend most sensitively on density. We present a study of the role of transition density in the DDT paradigm [1]. We apply a theoretical framework for statistically studying systematic effects using two-dimensional simulations that begin with a central deflagration having randomized perturbations. The DDT occurs when any rising plumes reach a specified density. We find a quadratic dependence of Fe-group yield on the log of DDT density. Assuming the DDT density depends on metallicity, we find the 56Ni yield decreases 0.067±0.004M⊙ for a 1 Z⊙ increase in metallicity.Item Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Progenitor 22Ne Content on Dynamics(2009-08-20) Townsley, Dean M.; Jackson, Aaron P.; Calder, Alan C.; Chamulak, David A.; Brown, Edward F.; Timmes, F. X.; University of Alabama TuscaloosaWe present a theoretical framework for formal study of systematic effects in supernovae Type Ia (SNe Ia) that utilizes two-dimensional simulations to implement a form of the deflagration–detonation transition (DDT) explosion scenario. The framework is developed from a randomized initial condition that leads to a sample of simulated SNe Ia whose 56Ni masses have a similar average and range to those observed, and have many other modestly realistic features such as the velocity extent of intermediate-mass elements. The intended purpose is to enable statistically well defined studies of both physical and theoretical parameters of the SNe Ia explosion simulation. We present here a thorough description of the outcome of the SNe Ia explosions produced by our current simulations. A first application of this framework is utilized to study the dependence of the SNe Ia on the 22Ne content, which is known to be directly influenced by the progenitor stellar population’s metallicity. Our study is very specifically tailored to measure how the 22Ne content influences the competition between the rise of plumes of burned material and the expansion of the star before these plumes reach DDT conditions. This influence arises from the dependence of the energy release, progenitor structure, and laminar flame speed on 22Ne content. For this study, we explore these three effects for a fixed carbon content and DDT density. By setting the density at which nucleosynthesis takes place during the detonation phase of the explosion, the competition between plume rise and stellar expansion controls the amount of material in nuclear statistical equilibrium (NSE) and therefore 56Ni produced. Of particular interest is how this influence of 22Ne content compares to the direct modification of the 56Ni mass via the inherent neutron excess as discussed by Timmes et al. Although the outcome following from any particular ignition condition can change dramatically with 22Ne content, with a sample of 20 ignition conditions we find that the systematic change in the expansion of the star prior to detonation is not large enough to compete with the dependence discussed by Timmes et al. In fact, our results show no statistically significant dependence of the predetonation expansion on 22Ne content, pointing to the morphology of the ignition condition as being the dominant dynamical driver of the 56Ni yield of the explosion. However, variations in the DDT density, which were specifically excluded here, are also expected to be important and to depend systematically on 22Ne content.Item On Measuring the Metallicity of a Type Ia Supernova's Progenitor(2016-06-10) Miles, Broxton J.; Van Rossum, Daniel R.; Townsley, Dean M.; Timmes, F. X.; Jackson, Aaron P.; Calder, Alan C.; Brown, Edward F.; University of Alabama TuscaloosaItem On Silicon Group Elements Ejected By Supernovae Type IA(2014-06-01) De, Soma; Timmes, F. X.; Brown, Edward M.; Calder, Alan C.; Townsley, Dean M.; Athanassiadou, Themis; Chamulak, David A.; Hawley, Wendy; Jack, Dennis; University of Alabama TuscaloosaThere is evidence that the peak brightness of a Type Ia supernova is affected by the electron fraction Ye 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 Ye at explosion from the abundances recovered from an observed spectra. We show that measurement of 28Si, 32S, 40Ca, and 54Fe abundances can be used to construct Ye in the silicon-rich regions of the supernovae. If these four abundances are determined exactly, they are sufficient to recover Ye 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 28Si abundance is insensitive to Ye, the 32S abundance has a nearly linear trend with Ye, and the 40Ca abundance has a nearly quadratic trend with Ye. We verify these trends with post-processing of one-dimensional models and show that these trends are reflected in the model’s synthetic spectra.Item On Variations of the Brightness of Type Ia Supernovae With the Age of the Host Stellar Population(2011) Krueger, Brendan K.; Jackson, Aaron P.; Calder, Alan C.; Townsley, Dean M.; Brown, Edward F.; Timmes, F.X.; University of Alabama TuscaloosaRecent observational studies of type Ia supernovae (SNeIa) suggest correlations between the brightness of an event and properties of the host galaxy that appear to involve the age of the progenitor population. One way to influence the explosion systematically is through the central density at ignition, which is determined by the mass of the white dwarf before the onset of accretion, the white dwarf cooling time (prior to the onset of accretion), the subsequent accretion history, and neutrino losses. The dependence of the central density on cooling time connects the central density to the age of the progenitor and therefore the average stellar age of the host galaxy. We find that with increased progenitor central density, production of Fe-group material does not change but production of 56Ni decreases, which we attribute to a higher rate of neutronization occurring at higher density. These results offer an explanation for the observation of dimmer SNeIa in galaxies with an older stellar population. We also demonstrate a strong dependence of the 56Ni yield in our results on the morphological structure of the burning front during the early deflagration, suggesting that a statistical ensemble of simulations is necessary when studying the systematics of SNeIa.Item Power-Law Wrinkling Turbulence-Flame Interaction Model for Astrophysical Flames(2014-04-01) Jackson, Aaron P.; Townsley, D. M.; Calder, Alan C.; University of Alabama TuscaloosaWe extend a model for turbulence–flame interactions (TFI) to consider astrophysical flames with a particular focus on combustion in Type Ia supernovae. The inertial range of the turbulent cascade is nearly always under-resolved in simulations of astrophysical flows, requiring the use of a model in order to quantify the effects of subgrid-scale wrinkling of the flame surface. We provide implementation details to extend a well-tested TFI model to low-Prandtl number flames for use in the compressible hydrodynamics code flash. A local, instantaneous measure of the turbulent velocity is calibrated for flash and verification tests are performed. Particular care is taken to consider the relation between the subgrid rms turbulent velocity and the turbulent flame speed, especially for high-intensity turbulence where the turbulent flame speed is not expected to scale with the turbulent velocity. Finally, we explore the impact of different TFI models in full-star, three-dimensional simulations of Type Ia supernovae.Item A Tracer Method for Computing Type Ia Supernova Yields: Burning Model Calibration, Reconstruction of Thickened Flames, and Verification for Planar Detonations(2016-07) Townsley, D. M.; Miles, Broxton J.; Timmes, F. X.; Calder, Alan C.; Brown, Edward F.; University of Alabama TuscaloosaWe refine our previously introduced parameterized model for explosive carbon–oxygen fusion during thermonuclear Type Ia supernovae (SNe Ia) by adding corrections to post-processing of recorded Lagrangian fluid-element histories to obtain more accurate isotopic yields. Deflagration and detonation products are verified for propagation in a medium of uniform density. A new method is introduced for reconstructing the temperature– density history within the artificially thick model deflagration front. We obtain better than 5% consistency between the electron capture computed by the burning model and yields from post-processing. For detonations, we compare to a benchmark calculation of the structure of driven steady-state planar detonations performed with a large nuclear reaction network and error-controlled integration. We verify that, for steady-state planar detonations down to a density of 5 × 106 g cm−3 , our post-processing matches the major abundances in the benchmark solution typically to better than 10% for times greater than 0.01 s after the passage of the shock front. As a test case to demonstrate the method, presented here with post-processing for the first time, we perform a two-dimensional simulation of a SN Ia in the scenario of a Chandrasekhar-mass deflagration–detonation transition (DDT). We find that reconstruction of deflagration tracks leads to slightly more complete silicon burning than without reconstruction. The resulting abundance structure of the ejecta is consistent with inferences from spectroscopic studies of observed SNe Ia. We confirm the absence of a central region of stable Fe-group material for the multidimensional DDT scenario. Detailed isotopic yields are tabulated and change only modestly when using deflagration reconstruction.Item Turbulence-Flame Interaction on the Early Evolution of Flames in Type Ia Supernovae(2011) Jackson, Aaron P.; Calder, Alan C.; Townsley, Dean M.; Chamulak, David A.; Brown, Edward F.; Timmes, F.X.; University of Alabama TuscaloosaItem Type Ia Supernova Explosions From Hybrid Carbon-Oxygen-Neon White Dwarf Progenitors(2016-11-20) Willcox, Donald E.; Townsley, Dean M.; Calder, Alan C.; Denissenkov, Pavel A.; Herwig, Falk; University of Alabama TuscaloosaMotivated by recent results in stellar evolution that predict the existence of hybrid white dwarf (WD) stars with a C–O core inside an O–Ne shell, we simulate thermonuclear (Type Ia) supernovae from these hybrid progenitors. We use the FLASH code to perform multidimensional simulations in the deflagration-to-detonation transition (DDT) explosion paradigm. Our hybrid progenitor models were produced with the MESA stellar evolution code and include the effects of the Urca process, and we map the progenitor model to the FLASH grid. We performed a suite of DDT simulations over a range of ignition conditions consistent with the progenitor’s thermal and convective structure assuming multiple ignition points. To compare the results from these hybrid WD stars to previous results from C–O WDs, we construct a set of C–O WD models with similar properties and similarly simulate a suite of explosions. We find that despite significant variability within each suite, trends distinguishing the explosions are apparent in their Ni 56 yields and the kinetic properties of the ejecta. We compare our results with other recent work that studies explosions from these hybrid progenitors.