  Since the discovery of hypervelocity stars in 2005, it has been widely
believed that only the disruption of a binary system by a supermassive black
hole at the Galactic center (GC), that is, the so-called Hills mechanism, is
capable of accelerating stars to beyond the Galactic escape velocity. In the
meantime, however, driven by the Gaia space mission, there is mounting evidence
that many of the most extreme high-velocity early-type stars at high Galactic
latitudes do originate in the Galactic disk and not in the GC. Moreover, the
ejection velocities of these extreme disk-runaway stars exceed the predicted
limits of the classical scenarios for the production of runaway stars. Based on
proper motions from the Gaia early data release 3 and on recent and new
spectrophotometric distances, we studied the kinematics of 30 such extreme
disk-runaway stars, allowing us to deduce their spatial origins in and their
ejection velocities from the Galactic disk with unprecedented precision. Only
three stars in the sample have past trajectories that are consistent with an
origin in the GC, most notably S5-HVS1, which is the most extreme object in the
sample by far. All other program stars are shown to be disk runaways with
ejection velocities that sharply contrast at least with classical ejection
scenarios. They include HVS5 and HVS6, which are both gravitationally unbound
to the Milky Way. While most stars originate from within a galactocentric
radius of 15kpc, which corresponds to the observed extent of the spiral arms, a
group of five stars stems from radii of about 21-29kpc. This indicates a
possible link to outer Galactic rings and a potential origin from infalling
satellite galaxies.
