The fastest stars in the Galaxy

We report a spectroscopic search for hypervelocity white dwarfs (WDs) that are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear explosions. Candidates selected Gaia data with high tangential velocities blue colors. find six new runaways, including four stars radial (RVs) $>1000\,\rm km\,s^{-1}$ total space $\gtrsim 1300\,\rm km\,s^{-1}$. These most likely the surviving donors double-degenerate binaries in which other WD exploded. The two objects have lower minimum velocities, 600\,\rm km\,s^{-1}$, may formed through different mechanism, such as pure deflagration of Iax supernova. fastest hotter smaller than previously known "D$^6$ stars," effective temperatures ranging $\sim$20,000 to $\sim$130,000 K radii $\sim 0.02-0.10\,R_{\odot}$. Three these carbon-dominated atmospheres, one has helium-dominated atmosphere. Two RVs $-1694$ $-2285\rm \,km\,s^{-1}$ -- systemic stellar ever measured. Their inferred birth 2200-2500\,\rm imply both WDs progenitor binary had masses $>1.0\,M_{\odot}$. observed suggest dominant fraction population comes whose mass significantly exceeds Chandrasekhar limit. However, nearest faintest D$^6$ lowest masses, suggesting observational selection effects favor rarer, higher-mass stars. A significant fainter low-mass still await discovery. infer rate is consistent SN rate. poorly constrained, however, because luminosities lifetimes $\rm D^6$ uncertain.