The Gaia Ultracool Dwarf Sample. I. Known L and T dwarfs and the first
  Gaia data release
  We identify and investigate known ultracool stars and brown dwarfs that are
being observed or indirectly constrained by the Gaia mission. These objects
will be the core of the Gaia ultracool dwarf sample composed of all dwarfs
later than M7 that Gaia will provide direct or indirect information on. We
match known L and T dwarfs to the Gaia first data release, the Two Micron All
Sky Survey and the Wide-field Infrared Survey Explorer AllWISE survey and
examine the Gaia and infrared colours, along with proper motions, to improve
spectral typing, identify outliers and find mismatches. There are 321 L and T
dwarfs observed directly in the Gaia first data release, of which 10 are later
than L7. This represents 45 % of all the known LT dwarfs with estimated Gaia G
magnitudes brighter than 20.3 mag. We determine proper motions for the 321
objects from Gaia and the Two Micron All Sky Survey positions. Combining the
Gaia and infrared magnitudes provides useful diagnostic diagrams for the
determination of L and T dwarf physical parameters. We then search the
Tycho-Gaia astrometric solution Gaia first data release subset to find any
objects with common proper motions to known L and T dwarfs and a high
probability of being related. We find 15 new candidate common proper motion
systems.
