# 8.3.5 Tycho2 merged with TDSC

Tycho-2

Reference paper:
Høg et al. 2000

Original catalogue:
CDS ftp://cdsarc.u-strasbg.fr/pub/cats/I/259/

The Tycho-2 Catalogue is an astrometric reference catalogue containing positions and proper motions as well as two-colour photometric data for the 2.5 million brightest stars in the sky. The Tycho-2 positions and magnitudes are based on precisely the same observations as the original Tycho Catalogue (hereafter Tycho-1) collected by the star mapper of the ESA Hipparcos satellite, but Tycho-2 is much bigger and slightly more precise, owing to a more advanced reduction technique. Components of double stars with separations down to 0.8 arcsec are included. Proper motions precise to about 2.5  mas yr${}^{-1}$are given as derived from a comparison with the Astrographic Catalogue and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most applications Tycho-1, as well as the ACT and TRC catalogues based on Tycho-1.

TDSC

Reference paper:
Fabricius et al. 2002

The authors report the discovery of 13 251 visual double stars, mostly with separations between 0.3 and 1 arcsec, from a dedicated re-reduction of the Tycho data from the star mapper of the ESA Hipparcos satellite. The new doubles are combined with 18 160 WDS systems identified in the Tycho-2, and 1220 new Tycho-2 doubles, to form the Tycho Double Star Catalogue, TDSC, a catalogue of absolute astrometry and BT, VT photometry for 66 219 components of 32 631 double and multiple star systems. They also include results for 32 263 single components for systems unresolved in TDSC, and a supplement gives Hipparcos and Tycho-2 data for 4777 additional components. The TDSC thus contains a total of 103 259 entries.

## Data curation and catalogue preparation

The merging process of Tycho-2 with TDSC (including its supplement) was done as follows :

• Tycho-2 astrometry was updated when new astrometry was available in TDSC (checking if the posflg column available in TDSC is set to ‘N’).

• If the pair is not resolved in Tycho-2, then the Tycho-2 entry is replaced by the two new.

The merged catalogue (Tycho2TDSCmerge) now contains 2 561 887 sources (compared to the Tycho-2 2 539 913 sources). The number of updated sources is 31 592, the number of added sources is 21 974.

## Cross-match algorithm

According to the definition used in this work, Tycho2TDSCmerge is a sparse catalogue. In the cross-match algorithm, Tycho2TDSCmerge is thus the leading catalogue, this means that counterparts for Tycho2TDSCmerge objects are searched for in Gaia.
For this catalogue the special treatment to account for the small issues in Gaia astrometry, especially for bright stars, was applied.