Hipparcos-2 BestNeighbour table lists each matched external catalogue object with its best
neighbour in Gaia.
source_id : Unique Gaia source identifier (long)
Unique identifier of the Gaia source, the attribute corresponds to gaia_source.source_id
original_ext_source_id : Original External Catalogue source identifier (long)
The unique source identifier in the original External catalogue.
angular_distance : Angular Distance between the two sources (double, Angle[arcsec])
Angular distance between a Gaia source and its nearest neighbour in the External Catalogue
number_of_neighbours : Number of neighbours in Gaia Catalogue (int)
Number of sources in the Gaia Catalogue which match the External Catalogue source within position errors.
The identifiers of all the neighbours can be found in the Neighbourhood table.
gaia_astrometric_params : Number of Gaia astrometric params used (short)
This field indicates the number of Gaia astrometric parameters which were available in Gaia.
The field is set to 2 when only RA and DEC where available, while is set to 5 when RA, DEC,
PMRA, PMDEC and PARALLAX are available and thus used to propagate a Gaia source position to the External Catalogue source coordinates epoch.