14.5.20 tycho2_neighbourhood

Tycho-2 Neighbourhood table includes all good neighbours for each matched external catalogue object. A good neighbour for a given external catalogue object is a nearby object in Gaia whose position is compatible (within position errors) with the target.

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 good neighbours in the External Catalogue

score : Score of neighbours (double)

The score is a figure of merit based on geometric distance and local density of the external catalogue: the higher the score, the more probable the match is.

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.