Zero Impact Parameter White Dwarf Collisions in FLASH

TL;DR

This study uses FLASH simulations to analyze zero impact parameter white dwarf collisions, revealing insights into nucleosynthesis and shock dynamics, but highlights challenges in achieving numerical convergence for precise elemental predictions.

Contribution

It provides the first systematic exploration of white dwarf collisions with detailed resolution analysis and discusses the implications for nucleosynthesis predictions.

Findings

Higher resolution models show trends in nickel and silicon production.

Collision models produce significant unburned carbon and oxygen.

Shock regions reach nuclear statistical equilibrium in more energetic collisions.

Abstract

We systematically explore zero impact parameter collisions of white dwarfs with the Eulerian adaptive grid code FLASH for 0.64+0.64 M$_{\odot}$ and 0.81+0.81 M$_{\odot}$ mass pairings. Our models span a range of effective linear spatial resolutions from 5.2$\times10^{7}$ to 1.2$\times10^{7}$ cm. However, even the highest resolution models do not quite achieve strict numerical convergence, due to the challenge of properly resolving small-scale burning and energy transport. The lack of strict numerical convergence from these idealized configurations suggest that quantitative predictions of the ejected elemental abundances that are generated by binary white dwarf collision and merger simulations should be viewed with caution. Nevertheless, the convergence trends do allow some patterns to be discerned. We find that the 0.64+0.64 M$_{\odot}$ head-on collision model produces 0.32 M$_{\odot}$ of \nickel[56] and 0.38 M$_{\odot}$ of \silicon[28], while the 0.81+0.81 M$_{\odot}$ head-on collision model produces 0.39 M$_{\odot}$ of \nickel[56] and 0.55 M$_{\odot}$ of \silicon[28] at the highest spatial resolutions. Both mass pairings produce $\sim$0.2 M$_{\odot}$ of unburned \carbon[12]+\oxygen[16]. We also find the 0.64+0.64 M$_{\odot}$ head-on collision begins carbon burning in the central region of the stalled shock between the two white dwarfs, while the more energetic \hbox{0.81+0.81 M$_{\odot}$} head-on collision raises the initial post-shock temperature enough to burn the entire stalled shock region to nuclear statistical equilibrium.