14.5.19 tycho2_best_neighbour

Tycho-2 table lists each matched external catalogue object with its best neighbour in Gaia.

Columns description:

source_id : Unique Gaia source identifier (long)

A unique single numerical identifier of the source obtained from gaia_source (for a detailed description see gaia_source.source_id)

original_ext_source_id : Original External Catalogue source identifier (string)

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 best neighbour in the External Catalogue

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.

tycho2_oid : External Catalogue source identifier (long)

The additional numeric unique source identifier of the External catalogue, increasing with Declination.

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.