Indications on the binary nature of individual stars derived from a comparison of their HIPPARCOS proper motions with ground-based data I. Basic principles

摘要

We present a method which provides some information on the possible binarynature on an appararently single star. The method compares the instantaneouslymeasured HIPPARCOS proper motion with the long-term averaged, ground-basedproper motion or with the proper motion derived from old ground-based positionsand the HIPPARCOS position. Good sources for such ground based data are the FK5and the GC. If the proper-motion difference Delta-mu is statisticallysignificant with respect to its measuring error, the object is very probably adouble star. We call then the object a 'delta-mu binary'. If the proper-motiondifference is insignificant and if no other information on a binary nature ofthe object is available, we call such a star a 'single-star candidate'. Wepropose a quantitative test for the significance of the observed proper-motiondifference. The sensitivity of our method is high: For nearby stars at adistance of 10 pc, the measuring accuracy of the proper-motion difference,expressed as a velocity, is of the order of 50 m/s (basic FK5 stars) or 80 m/s(GC stars). At 100 pc, the mean error of the two-dimensional difference isstill 0.5 km/s or 0.8 km/s. For the FK5 stars, we provide indications on theprobable period of the delta-mu binaries. If we adopt an orbital period and amass-luminosity relation, we can use the observed velocity difference toestimate the separation and the magnitude difference between the two componentsof the binary. The present paper concentrates mainly on the basic principles ofthe method, but it provides also a few examples of delta-mu binaries and ofsingle-star candidates for illustration: Gamma UMa, Epsilon Eri, Iota Vir, 47UMa, Delta Pav.