The response of a helium white dwarf to an exploding Type Ia supernova

We conduct numerical simulations of the interacting ejecta from an exploding CO white dwarf (WD) with a He WD donor in the double-detonation scenario for Type Ia supernovae (SNe Ia), and study the possibility of exploding the companion WD. We also study the long time imprint of the collision on the supernova remnant. When the donor He WD has a low mass, M_WD = 0.2M⊙, it is at a distance of 0.08 R⊙ from the explosion, and helium is not ignited. The low-mass He WD casts an 'ejecta shadow' behind it. By evolving the ejecta for longer times, we find that the outer parts of the shadowed side are fainter and its boundary with the ambient gas is somewhat flat. More massive He WD donors, M_WD ≃ 0.4M⊙, must be closer to the CO WD to transfer mass. At a distance of a ≤ 0.045R⊙ helium is detonated and the He WD explodes, leading to a triple detonation scenario. In the explosion of the donor WD approximately 0.15M⊙ of unburned helium is ejected. This might be observed as a peculiar Type Ib supernova.