2 The Tycho catalogues

The observations from the Tycho experiment onboard the Hipparcos satellite were exploited to produce the Tycho-1 Catalogue (ESA 1997) of one million stars with positions and $B_{\rm T}$, $V_{\rm T}$ magnitudes. Many doubles with separations above 2 arcsec were resolved.

The Tycho-2 Catalogue (Høg et al. 2000b), which has superceded Tycho-1, is the result of a second data reduction, using more advanced and demanding techniques. The Tycho-2 Catalogue contains positions, proper motions and $B_{\rm T}$, $V_{\rm T}$ magnitudes for 2.5 million stars. The proper motions were derived from the Tycho-2 positions, the 100-year-old positions in the Astrographic Catalogue and 143 other astrometric catalogues.

A supplement to Tycho-2 contains stars from Tycho-1 and Hipparcos, including double star components, which are not in Tycho-2. Close Hipparcos doubles which are included, but unresolved, in Tycho-2 are, however, not part of the supplement. For completeness and to reduce ambiguities, we have included data from this supplement in the present work when cross referencing to the WDS, and the relevant entries are given in the TDSC Supplement. A second supplement to Tycho-2 contains Tycho-1 stars, also not in Tycho-2, which are suspected to be either ghosts produced by sidelobes or heavily disturbed detections. Here we have ignored this second supplement.

In Tycho-2, double stars with separations down to 0.8 arcsec were resolved in a dedicated double-star reduction process, carried out for only a small subset of the stars. The production of the Tycho-2 Catalogue was based on an input catalogue, allowing a field of radius 2.5 arcsec around each input position to be analysed. For the dedicated double star analysis to work, both components must be within this field. This sets an upper limit to the separation unless the components have separate entries in the input catalogue.

Now, a third catalogue, the Tycho Double Star Catalogue, complements Tycho-2 with respect to double stars. It presents accurate positions, proper motions, $B_{\rm T}$ and $V_{\rm T}$ photometry for $66\,219$ components of $32\,631$ visual double and multiple star systems, as well as results for $32\,263$ systems, unresolved by Tycho. In contrast to the system based WDS, where pairs of stars are listed, we give, as far as possible, results for the individual components, with one line per star. For 4777 additional components, mostly faint, which were not detected in Tycho-2, we give a supplement with the Hipparcos or Tycho-1 positions, including the Hipparcos double star components, copied across from the Tycho-2 supplement. The TDSC contains a total of $103\,259$ entries, including the supplement. The source of astrometry is indicated for all stars by a flag.

Observations spread over 3 years are combined to yield mean astrometry for that period, allowing only for the (linear) proper motion known in the input catalogue. Any additional motion will cause smearing of the image, and doubles with a short orbital period will therefore not be detected.

Due to the nature of the Tycho data, only systems with modest magnitude differences can be detected. As is shown in Fig. 1, it rarely exceeds 2 mag. Most systems have primaries of about $V_{\rm T}$ = 11.5 mag, and seldom fainter than 12 mag. For separations below 1 arcsec, the conditions are difficult, and the limit is brighter. It may also be noticed that we have the best conditions for separations between 1 and 3 arcsec. This is due to the size of the input fields in the Tycho-2 data base. The Tycho separations go from 0.29 arcsec to 4680 arcsec. The larger separations are based on normal treatment of two separate input fields. The supplement also contains smaller separations, down to 0.13 arcsec, for pairs involving a Tycho star and a (faint) Hipparcos star.

Figure 1: The distribution of magnitude difference and actual magnitude is indicated for the smaller separations.