Zero Impact Parameter White Dwarf Collisions in FLASH
  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. 
Wendy Hawley, Themis Athanassiadou, Francis Timmes
astro-ph.SR