Gaia DR2 white dwarfs in the Hercules stream
  We analyzed the velocity space of the thin and thick-disk Gaia white dwarf
population within 100 pc looking for signatures of the Hercules stellar stream.
We aimed to identify those objects belonging to the Hercules stream and, by
taking advantage of white dwarf stars as reliable cosmochronometers, to derive
a first age distribution. We applied a kernel density estimation to the $UV$
velocity space of white dwarfs. For the region where a clear overdensity of
stars was found, we created a 5-D space of dynamic variables. We applied a
hierarchichal clustering method, HDBSCAN, to this 5-D space, identifying those
white dwarfs that share similar kinematic characteristics. Finally, under
general assumptions and from their photometric properties, we derived an age
estimate for each object. The Hercules stream was firstly revealed as an
overdensity in the $UV$ velocity space of the thick-disk white dwarf
population. Three substreams were then found: Hercules $a$ and Hercules $b$,
formed by thick-disk stars with an age distribution peaked $4\,$Gyr in the past
and extended to very old ages; and Hercules $c$, with a ratio of 65:35
thin:thick stars and a more uniform age distribution younger than 10 Gyr.
