Central Density Effects on
White Dwarf Supernovae


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Contact: F.X.Timmes
my one page vitae,
full vitae,
research statement, and
teaching statement.

Evaluating Systematic Dependencies Of Type Ia Supernovae: The Influence Of Central Density (2012)
In this article we 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 $^{56}$Ni, 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 $^{56}$Ni 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 $^{56}$Ni in the models, including the lack of a consistent centrally located deficit of $^{56}$Ni, 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.

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On Variations Of The Brightness Of Type Ia Supernovae With The Age Of The Host Stellar Population (2010)
Recent observational studies of type Ia supernovae (SNeIa) suggest correlations between the peak brightness of an event and the age of the progenitor stellar population. This trend likely follows from properties of the progenitor white dwarf (WD), such as central density, that follow from properties of the host stellar population.

letter we present a statistically well-controlled, systematic study utilizing a suite of multi-dimensional SNeIa simulations investigating the influence of central density of the progenitor WD on the production of Fe-group material, particularly radioactive $^{56}$Ni, which powers the light curve. We find that on average, as the progenitor's central density increases, production of Fe-group material does not change but production of $^{56}$Ni decreases. We attribute this result to a higher rate of neutronization at higher density. The central density of the progenitor is determined by the mass of the WD and the cooling time prior to the onset of mass transfer from the companion, as well as the subsequent accretion heating and neutrino losses. The dependence of this density on cooling time, combined with the result of our central density study, offers an explanation for the observed age-luminosity correlation: a longer cooling time raises the central density at ignition thereby producing less $^{56}$Ni and thus a dimmer event. While our ensemble of results demonstrates a significant trend, we find considerable variation between realizations, indicating the necessity for averaging over an ensemble of simulations to demonstrate a statistically significant result.