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Double detonations in double white dwarf (WD) binaries undergoing unstablemass transfer have emerged in recent years as one of the most promising Type Iasupernova (SN Ia) progenitor scenarios. One potential outcome of this"dynamically driven double-degenerate double-detonation" (D^6) scenario is thatthe companion WD survives the explosion and is flung away with a velocity equalto its > 1000 km/s pre-SN orbital velocity. We perform a search for thesehypervelocity runaway WDs using Gaia's second data release. In this paper, wediscuss seven candidates followed up with ground-based instruments. Threesources are likely to be some of the fastest known stars in the Milky Way, withtotal Galactocentric velocities between 1000 and 3000 km/s, and are consistentwith having previously been companion WDs in pre-SN Ia systems. However,although the radial velocity of one of the stars is > 1000 km/s, the radialvelocities of the other two stars are puzzlingly consistent with 0. Thecombined five-parameter astrometric solutions from Gaia and radial velocitiesfrom follow-up spectra yield tentative 6D confirmation of the D^6 scenario. Thepast position of one of these stars places it within a faint, old SN remnant,further strengthening the interpretation of these candidates as hypervelocityrunaways from binary systems that underwent SNe Ia.

Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae