J-PLUS: Discovery and characterisation of ultracool dwarfs using Virtual
  Observatory tools
  Ultracool dwarfs (UCDs) are objects with spectral types equal or later than
M7. Most of them have been discovered using wide-field imaging surveys. The
Virtual Observatory (VO) has proven to be of great utility to efficiently
exploit these astronomical resources. We aim to validate a VO methodology
designed to discover and characterize UCDs in the J-PLUS photometric survey.
J-PLUS is a multiband survey carried out with the wide angle T80Cam optical
camera mounted on the 0.83-m telescope JAST/T80 in the Observatorio
Astrof\'isico de Javalambre. In this work we make use of the Internal Data
Release (IDR) covering 528 deg$^2$. We complement J-PLUS photometry with other
catalogues in the optical and IR using VOSA, a VO tool that estimates physical
parameters from the spectral energy distribution fitting to collections of
theoretical models. Objects identified as UCDs are distinguished from
background M giants and highly reddened stars using parallaxes and proper
motions from Gaia DR2. We identify 559 UCDs, ranging from i=16.2 to 22.4 mag,
of which 187 are candidate UCDs not previously reported in the literature. This
represents an increase in the number of known UCDs of about 50% in the studied
region of the sky, particularly at the faint end of our sensitivity, which is
interesting as reference for future wide and deep surveys such as Euclid. Three
candidates constitute interesting targets for exoplanet surveys because of
their proximity (<40 pc). We also analyze the kinematics of UCDs in our
catalogue and find evidence that it is consistent with a Galactic thin-disk
population, except for 6 objects that might be members of the thick disk. The
results obtained validate the proposed methodology, which will be used in
future J-PLUS and J-PAS releases. Considering the region of the sky covered by
the IDR used, we foresee to discover 3,000-3,500 new UCDs at the end of the
J-PLUS project.
