How many Hipparcos Variability-Induced Movers are genuine binaries? ^*,**

¹ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
² Department of Astrophysical Sciences, Princeton University, Princeton NJ 08543-1001, USA
³ Universities Space Research Association, Division of Astronomy and Space Physics, 300 D Street SW, Washington D.C. 20024, USA
⁴ Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁵ Département de Physique, Université de Mons Hainaut, Avenue du Champ de Mars 8, 7000 Mons, Belgium
⁶ US Naval Observatory, Washington D.C., USA

Corresponding author: D. Pourbaix, pourbaix@astro.ulb.ac.be

Received: 20 September 2002
Accepted: 5 December 2002

Abstract

Hipparcos observations of some variable stars, and especially of long-period (e.g. Mira) variables, reveal a motion of the photocenter correlated with the brightness variation ( variability-induced mover – VIM), suggesting the presence of a binary companion. A re-analysis of the Hipparcos photometric and astrometric data does not confirm the VIM solution for 62 among the 288 VIM objects (21%) in the Hipparcos catalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables (LPV). The effect of a revised chromaticity correction, which accounts for the color variations along the light cycle, was then investigated. It is based on “instantaneous”  color indices derived from Hipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged as VIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM after this improved chromaticity correction is applied. This dramatic decrease in the number of VIM solutions is not surprising, since the chromaticity correction applied by the Hipparcos reduction consortia was based on a fixed  color. Astrophysical considerations lead us to adopt a more stringent criterion for accepting a VIM solution (first-kind risk of 0.27% instead of 10% as in the Hipparcos catalogue). With this more severe criterion, only 27 LPV stars remain VIM, thus rejecting 161 of the 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.