We report a spectroscopic search for hypervelocity white dwarfs (WDs) that
are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear
explosions. Candidates are selected from Gaia data with high tangential
velocities and blue colors. We find six new runaways, including four stars with
radial velocities (RVs) >1000kms−1 and total space velocities
≳1300kms−1. These are most likely the surviving donors from
double-degenerate binaries in which the other WD exploded. The other two
objects have lower minimum velocities, ≳600kms−1, and may
have formed through a different mechanism, such as pure deflagration of a WD in
a Type Iax supernova. The four fastest stars are hotter and smaller than the
previously known "D6 stars," with effective temperatures ranging from
∼20,000 to ∼130,000 K and radii of ∼0.02−0.10R⊙. Three
of these have carbon-dominated atmospheres, and one has a helium-dominated
atmosphere. Two stars have RVs of −1694 and −2285kms−1 -- the
fastest systemic stellar RVs ever measured. Their inferred birth velocities,
∼2200−2500kms−1, imply that both WDs in the progenitor binary
had masses >1.0M⊙. The high observed velocities suggest that a
dominant fraction of the observed hypervelocity WD population comes from
double-degenerate binaries whose total mass significantly exceeds the
Chandrasekhar limit. However, the two nearest and faintest D6 stars have the
lowest velocities and masses, suggesting that observational selection effects
favor rarer, higher-mass stars. A significant population of fainter low-mass
runaways may still await discovery. We infer a birth rate of D6 stars that
is consistent with the SN Ia rate. The birth rate is poorly constrained,
however, because the luminosities and lifetimes of D6 stars are
uncertain.Comment: 26 pages, 17 figures. Accepted to OJ