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Collisions of white dwarfs (WDs) have recently been invoked as a possiblemechanism for type Ia supernovae (SNIa). A pivotal feature for the viability ofWD collisions as SNIa progenitors is that a significant fraction of the mass ishighly compressed to the densities required for efficient $^{56}$Ni productionbefore the ignition of the detonation wave. Previous studies have predominantlyemployed model WDs composed entirely of carbon-oxygen (CO), whereas WDs areexpected to have a non-negligible helium envelope. Given that helium is moresusceptible to explosive burning than CO under the conditions characteristic ofWD collision, a legitimate concern is whether or not early time He detonationignition can translate to early time CO detonation, thereby drasticallyreducing $^{56}$Ni synthesis. We investigate the role of He in determining thefate of WD collisions by performing a series of two-dimensional hydrodynamicscalculations. We find that a necessary condition for non-trivial reduction ofthe CO ignition time is that the He detonation birthed in the contact regionsuccessfully propagates into the unshocked shell. We determine the minimal Heshell mass as a function of the total WD mass that upholds this condition.Although we utilize a simplified reaction network similar to those used inprevious studies, our findings are in good agreement with detailedinvestigations concerning the impact of network size on He shell detonations.This allows us to extend our results to the case with more realistic burningphysics. Based on the comparison of these findings against evolutionarycalculations of WD compositions, we conclude that most, if not all, WDcollisions will not be drastically impacted by their intrinsic He components.

Can helium envelopes change the outcome of direct white dwarf collisions?