3 Stellar contents of the TDSC

 In the Tycho Double Star Catalogue, we have included three groups of stars:

• Tycho double star solutions;
• Tycho-2 stars identified in WDS;
• Tycho-2 pairs separated by less than 10 arcsec.

3.1 Tycho double star solutions

 

New double star solutions were attempted for all Tycho-2 stars, looking for a companion within the field of 2.5 arcsec radius. The method used for Tycho-2, as described in Høg et al. (2000a), was adopted with a few modifications. First of all, stars which did not look promising after a few iterations were deemed single and no further analyses were attempted. This was necessary to save computing time, but some close doubles may have been lost. Secondly, new criteria were introduced regarding when to terminate the loop of iterations, instead of the fixed number of iterations in Tycho-2. This was partly introduced for efficiency and partly because some systems were converging very slowly. As a consequence there can be minor discrepancies between the new solutions and the original Tycho-2 solutions. Finally, the criteria, regarding which solutions to accept and which to discard, were revised. Smaller separations than the very conservative limit of 0.8 arcsec used in Tycho-2 were accepted, but at the same time a separation dependent limit on the signal-to-noise ratio was introduced. The smallest accepted separation in the Tycho solutions was 0.29 arcsec.

A few stars which were excluded from Tycho-2 have now been successfully resolved, and accepted for the present catalogue. They are the pairs TDSC 28625, 42517 and 51915.

3.2 Identification of Tycho-2 stars in WDS

A cross reference list between WDS and Tycho-2 was presented by Mason et al. (2000b). It was, however, soon realised that a more complete, but perhaps less safe, list should be produced. This new list should not only list the primary components, but also sort out the individual components of multiple systems.

The WDS, being compiled from hundreds of sources, is very inhomogeneous and there are occasional ambiguities regarding what pairs constitute one system, or how to designate some individual components. We adopted, as a main rule, the definition that all pairs having the same position field in WDS constitute one system. For each such system, we looked up all stars in Tycho-2 (including the supplement) within a radius given by the maximum separation in the system plus 1.5 or 3 arcmin, depending on the quality of the WDS astrometry. We then looked for pairs of Tycho-2 stars which had the right separation and position angle (except perhaps for $\pm 180 \hbox{$^\circ$ }$) at the epoch in question. For the majority of systems, this worked fine, but a significant fraction of the cases could not be handled automatically. Often, the designations were inconsistent; or a very bright double was apparently not in Tycho-2 at all; or the same Tycho-2 star appeared in more than one system. All in all, we identified more than $48\,000$ components in more than $23\,000$ systems with separations between 0.13 and 4680 arcsec. These numbers include 4000 new Tycho pairs and also 4800 Hipparcos and Tycho-1 components, copied across from the Tycho-2 supplement.

In cross referencing Tycho-2 with WDS, there were almost $29\,000$ systems where we found only one Tycho-2 star in the vicinity. This Tycho-2 star is then a likely identification for typically either the A component or the photocentre of A and B. We have included these stars in the TDSC.

3.3 Tycho-2 doubles missing in WDS

The last group of stars consists of Tycho-2 stars which were not found in the version of WDS we used, and which are not part of a double star solution, but which have a partner in Tycho-2 within 10 arcsec. There were 359 such doubles, some of which have recently been included in WDS from the Washington Fundamental Catalog. The distance of 10 arcsec was chosen because doubles closer than this limit are often not resolved in scans of Schmidt plates.

3.4 The TDSC supplement

The TDSC Supplement contains Hipparcos and Tycho-1 data for 4777, mostly faint, resolved or unresolved components of WDS systems, which were not detected in Tycho-2. Normally, these additional components complement single components in the main TDSC. Only 316 systems in the supplement have no components in the main catalogue.

3.5 Incorporation into the Washington Double Star Catalog

Following the new Tycho solutions and the new cross identification list, an additional $25\,235$ measures (i.e. pairs), of which $13\,080$are resolved here for the first time, were incorporated into the WDS. The WDS is continuously growing, and at the time of inclusion of the TDSC results, a few of our discoveries were no longer discoveries. All TDSC systems have a running number, $1\ldots65\,210$, in the TDSC. The subset of new systems have also a discovery designation assigned, when added to the WDS. The assignment of TDS discovery designations for the new systems begins where Mason et al. (2000b) leaves off, i.e., TDS1235. The four-digit discovery number has been adequate for all historical binary star work. However, the work here exceeds this. For the WDS we adopt the admittedly somewhat makeshift strategy where all TDS numbers beyond 9999 are assigned to the discovery designation "TDT'', beginning with "TDT0001''. Of the $12\,155$ measures of known systems, 891 are measures where the position angle differed by 30 degrees or more, or the separation differed by 30% when compared with more recent measures. The majority of these systems seem to have small separations. Of those with larger separations many do not have contemporary measures, so the Tycho-2 measure may not reflect an error, simply a difference. The relevant components of these 891 systems are flagged in the catalogue.