Re-processing the hipparcos transit data and intermediate astrometric dataof spectroscopic binaries - I. Ba, CH and Tc-poor S stars

Only 235 entries were processed as astrometric binaries with orbits in the Hipparcos and Tycho Catalogue (ESA 1997). However, the Intermediate Astrome tric Data (IAD) and Transit Data (TD) made available by ESA make it possibl e to re-process the stars that turned out to be spectroscopic binaries afte r the completion of the Catalogue. This paper illustrates how TD and IAD ma y be used in conjunction with the orbital parameters of spectroscopic binar ies to derive astrometric parameters. The five astrometric and four orbital parameters (not already known from the spectroscopic orbit) are derived by minimizing an objective function (chi(2)) with an algorithm of global opti mization. This code has been applied to 81 systems for which spectroscopic orbits became available recently and that belong to various families of che mically-peculiar red giants (namely, dwarf barium stars, strong and mild ba rium stars, CH stars, and Tc-poor S stars). Among these 81 systems, 23 yiel d reliable astrometric orbits. These 23 systems make it possible to evaluat e on real data the so-called "cosmic error" described by Wielen et al. (199 7), namely the fact that an unrecognized orbital motion introduces a system atic error on the proper motion. Comparison of the proper motion from the H ipparcos catalogue with the re-derived in the present work indicates that t he former are indeed far off the present value for binaries with periods in the range 3 to similar to 8 years. Hipparcos parallaxes of unrecognized sp ectroscopic binaries turn out to be reliable, except for systems with perio ds close to 1 year, as expected. Finally, we show that, even when a complet e orbital revolution was observed by Hipparcos, the inclination is unfortun ately seldom precise.