2. The selection of systems and orbits
2.1. The sources of data
According to the results of the simulation presented in Martin et al., 1997, the search of astrometric binaries likely candidates for a mass determination from the Hipparcos observations, must be restricted to pairs with orbital periods smaller than or equal to 30 years. Another selection based on the separation will be considered later. Four main sources of orbits were used to identify the potential systems :
The above sources are in practice largely redundant. The intersection of these four sources and that of the Hipparcos observing program combined with the limitation in period, yields at the end a set of 302 orbits for 191 different entries of the Hipparcos Catalogue. This constitutes the basic data set to be investigated.
2.2. Elimination of objects
Among these 191 objects, there are several systems which cannot be processed by the method of Paper I, because of the parasitic effect of a third component in the vicinity () of the central pair. This led to the rejection of 16 systems from the initial set. In addition, two more systems were eliminated as they were finally not successfully observed by the satellite (HIP 21088 and HIP 116191). After this step, we were left with 277 orbits for 173 objects.
There were in this sample 28 astrometric pairs for which the published orbits referred to the absolute motion of the photocentre on the sky, instead of the relative motion of the two components. In this case, nothing more could be done with the Hipparcos data and these stars had to be removed from the sample. Eventually, the useful sample numbers 242 orbits associated to 145 objects listed in 1by their HIP identifier.
2.3. Description of the subset of binaries
2.3.1. Statistical description
The first description proposed here aims to recognize the pairs for which the independent determination of the mass and intensity ratios seems possible, from those for which the values of the period and the separation will not allow this distinction. Fig. 1a reveals a non negligible population of interesting pairs with semi-major axes larger than 0:0025, but also with periods generally larger than 15 years. It shows also the difficulty, expected indeed, to find binaries with both large separations and small periods. Fig. 1b shows the distribution of the largest separation on the sky reached by every pair during a complete revolution (as long as the Hipparcos observations are concerned, this quantity is more representative than the semi-major axis).
2.3.2. An adapted characterization
As the sample of objects considered is not too large (compared to the whole set of about 12000 binaries for which an astrometric solution is published in the final Hipparcos Catalogue), it has been possible to define an 'identity card' of each selected star, allowing to illustrate the potential of Hipparcos in each case. In the following diagrams, each 'card' appears as a segment and four numbers, with the following meanings:
This information is displayed in Fig. 2. Independently, on each diagram three horizontal lines have been drawn and represent the theoretical thresholds in apparent separation beyond which it becomes possible to distinguish the photocentre and the Hippacentre of the star, for a system of 2 mag (bottom line), 10 mag (middle line) and 12 mag (top line). In connection with the results of the simulation of Paper I, the previous representation allows to estimate quickly what can be expected from the processing of each star, assuming that the orbit is perfectly known. However only about 25% of the published orbits are considered as 'very good' or 'definitive' according to the criteria of the Worley's catalog; this is probably the main reason why significant results could not be obtained for all of the stars considered here.
Table 1. HIP numbers of the 145 preselected systems
Table 2. The set of astrometric binaries for which a ground-based orbit has been used to reprocess the Hipparcos observations for determining the mass of the components. The columns give the Hipparcos, ADS and HD identifiers, the usual name, the semi-major axis of the relative orbit in arcsec and the orbital period in years.
Table 3. Same set as in Table 2, showing the Hipparcos results from the standard processing. The labels C, O, G, X refer to the sections of the Double and Multiple Systems Annex in which the solution has been placed. A blank in this column indicates that a single star solution has been adopted.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998