Binaries in the Hipparcos data: Keep digging, I. Search for binaries without `a priori' knowledge of their orbital elements: Application to barium stars

This work makes use of Hipparcos data to test the algorithms of (i) binary detection and (ii) orbital-parameters determination, which could possibly be used in the GAIA pipeline. The first item is addressed in this paper, whereas the second one is addressed in a companion paper by Pourbaix, Jancart, & Jorissen (2004, this volume). Here we test the ability of the algorithm to detect binaries from scratch, i.e., from the astrometric data without any a priori knowledge of the orbital elements. The Hipparcos Intermediate Astrometric Data of a complete sample of 163 barium stars (supposed to be all members of binary systems from theoretical considerations) are used as a test bench. When > 5 mas and P 5 mas and P 4000 d (11 y) can generally not be extracted from the Hipparcos IAD, which span only 3 y (see left panel of Fig. 1). Similarly, when < 5 mas, as have most barium stars (right panel of Fig. 1), the Hipparcos IAD are not precise enough to extract the orbital motion. An interesting astrophysical outcome of the present work is the list of barium stars shown to be astrometric binaries by the analysis of the Hipparcos IAD. They are listed in Table 1, with an estimate of their orbital period, as obtained from Fig. 2. Acknowledgments. A. J. and D. P. are Research Associates, F.N.R.S (Belgium). Financial support to this work was was provided through ESA/PRODEX Grants 90078 and 15152.