Runaway blue main-sequence stars at high Galactic latitudes. Target
  selection with Gaia and spectroscopic identification
  Motivated by the historical identification of runaway main-sequence (MS)
stars of early spectral type at high Galactic latitudes, we test the capability
of Gaia at identifying new such stars. We have selected ~2300 sources with Gaia
magnitudes of GBP - GRP < 0.05, compatible with the colors of low-extinction MS
stars earlier than mid-A spectral type, and obtained low-resolution optical
spectroscopy for 48 such stars. By performing detailed photometric and
spectroscopic analyses, we derive their atmospheric and physical parameters
(effective temperature, surface gravity, radial velocity, interstellar
reddening, spectrophotometric distance, mass, radius, luminosity, and age). The
comparison between spectrophotometric and parallax-based distances enables us
to disentangle the MS candidates from older blue horizontal branch (BHB)
candidates. We identify 12 runaway MS candidates, with masses between 2 and 6
Msun. Their trajectories are traced back to the Galactic disc to identify their
most recent Galactic plane crossings and the corresponding flight times. All 12
candidates are ejected from the Galactic disc within 2 to 16.5 kpc from the
Galactic center and possess flight times that are shorter than their
evolutionary ages, compatible with a runaway hypothesis. Three MS candidates
have ejection velocities exceeding 450 km/s, thus, appear to challenge the
canonical ejection scenarios for late B-type stars. The fastest star of our
sample also has a non-negligible Galactic escape probability if its MS nature
can be confirmed. We identify 27 BHB candidates, and the two hottest stars in
our sample are rare late O and early B type stars of low mass evolving towards
the white dwarf cooling sequence.
