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New ultracool dwarf neighbours within 20pc from Gaia DR2

The Gaia data release 2 (DR2) contains $>$6000 objects with parallaxes $(Plx+3\times{e\_Plx})>50$mas placing them within 20pc from the Sun. Since the expected numbers extrapolating the well-known 10pc census are much lower, nearby Gaia stars need a quality assessment. The 20pc sample of white dwarfs (WDs) had already been verified and completed with Gaia DR2. We aimed to check and complete the 20pc sample of ultracool dwarfs (UCDs) with spectral types $\gtrsim$M7 and given Gaia DR2 parallaxes. Dividing the Gaia DR2 20pc sample into subsamples of various astrometric and photometric quality, we studied their distribution on the sky, in the $M_G$ vs. $G$$-$$RP$ colour-magnitude diagram (CMD), and as a function of $G$ magnitude and total proper motion. After excluding 139 known WDs and 263 known UCDs from the CMD, we checked all remaining $\approx$3500 candidates with $M_G>14$mag (used to define UCDs in this study) for the correctness of their Gaia DR2 proper motions. For confirmed UCD candidates we estimated spectral types photometrically using Gaia and near-infrared absolute magnitudes and colours. We failed to confirm new WDs, but found 50 new UCD candidates not mentioned in three previous studies using Gaia DR2. They have relatively small proper motions and tangential velocities and are concentrated towards the Galactic plane. Half of them have spectral types in SIMBAD and/or previous non-Gaia distance estimates that placed them already within 20pc. For 20 of the 50 objects, we estimated photometric spectral types of M6-M6.5, slightly below the classical UCD spectral type limit. However, seven L4.5-L6.5, four L0-L1, five M8.5-M9.5, and three M7-M8 dwarfs can be considered as completely new UCDs discoveries within 20pc based on Gaia DR2. Four M6.5 and two L4.5 dwarfs have high membership probabilities (64%-99%) in the ARGUS, AB Doradus, or Carina Near young moving groups. (abbreviated)