Astron. Astrophys. 356, 141-145 (2000)

1. Introduction

The Hipparcos Double and Multiple Systems Annex (DMSA), as a part of the Hipparcos catalogue, is one of the major products of the ESA Hipparcos astrometric mission (ESA 1997). It comprises by far the most accurate astrometric and photometric information on double and multiple stars detected in various ways. Its Components Solution part contains the results on distinctly resolved components of visual systems, not exhibiting a measurable orbital motion over the 3.5 years duration of the active observation. The DMSA contains doubles, 182 triples and 8 quadruples, thus adding up to a total of resolved components.

Although this is not the largest set of multiple systems, the uniformity and high accuracy of data makes DMSA an important tool for binary stars investigations for a long time to come. Besides the astrometric data (positions, proper motions, parallaxes, angular separations and position angles), broad-band Hp magnitudes were given for all components, and the Tycho and magnitudes, somewhat similar to the Johnson magnitudes - for some of them. The and magnitudes were obtained with a separate instrument, Tycho, on board of the Hipparcos satellite resulting in the Tycho-1 Catalogue (ESA 1997, Vol. 4). Only less than half of the systems actually have the two-colour photometry from Tycho-1 in DMSA. There were two reasons for that. Firstly, the angular resolution of the Tycho-1 Catalogue (about 2.0 arcsec) was not matching that of the Hipparcos main instrument (0.1 arcsec). Hence, the majority of close binaries were not resolved in Tycho-1, and a photocentre solution was obtained in the best case. Secondly, some of the Hipparcos double stars were too faint for the Tycho instrument, its limiting magnitude being of the order  mag. Components with magnitude differences above 1.0 to 1.5 mag could not be resolved in Tycho-1 either.

It is important to know the colours of both components of a binary for some astrophysical studies. The lack of two-colour photometry in the Hipparcos catalogue for close doubles is a flaw in this respect. In this paper, we present the results of a new photometric solution for close Hipparcos double and triple stars, based on the original (raw) Tycho observational data, but processed in a very different and more advanced way.

Soon after the completion of the Tycho-1 Catalogue (ESA 1997) Hog et al. (1998) decided to carry out a second reduction of the Tycho data, applying a more advanced reduction technique. The photometry presented here is not part of the new Tycho-2 Catalogue, but it was made possible by the creation of the Identified Counts Data Base (ICDB) for more than 2.5 million stars in the course of the Tycho-2 reprocessing and catalogue construction (Hog et al. 1998, and Hog et al. 2000). The Tycho-2 Catalogue extends the Hipparcos/Tycho reference frame from 1.1 to 2.5 million stars by pushing the limiting magnitude fainter. This improvement of the limiting magnitude by about 0.4 mag was achieved owing to a much better method of data processing, namely a photon superposition for the whole mission . The photon superposition also gives better astrometric and photometric values for the fainter half of the original one million stars. Finally, a much better angular resolution was achieved.

The Tycho-2 Catalogue contains a few thousand close double systems with separations down to 0.8 arcsec. Many of them are not given in the Hipparcos catalogue, and some are actually new discoveries. The doubles with separations less than 2.5 arcsec were resolved in a special process, briefly discussed in Hog et al. (2000). It included a full astrometric and photometric solution for the components, based entirely on the Tycho ICDB dataset. Even though almost half of the stars were in Hipparcos, no external information was used in the solution in any way except for the general instrument calibration.

The present solution is different, because the exact positions of the Hipparcos resolved component are used. An astrometric solution is thus eliminated, making it much simpler than the routine Tycho-2 double star solution.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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