Binary statistics from Hipparcos data - a progress report

Abstract

The Hipparcos data are an important source for constraining the statistical distribution functions for the binaries in the general field. The present study uses the resolved binaries (separation 0.1-10 arcsec, magnitude-differences below 4 mag) to check directly the frequency of main-sequence binaries with (linear) separations 30-500 a.u. and mass-ratios 0.6-1.0. Complete Hipparcos samples have limiting apparent magnitudes brighter than mag 8, and contain therefore rather few stars. By modelling the completeness using the Tycho observations (complete to mag 10), we may increase substantially the number of useable Hipparcos stars while keeping a tolerable level of systematic uncertainty. The results, for stellar masses not much above solar, show binary frequencies typically a factor two above those usually assumed. Also, although the mass-ratio distribution function generally decreases towards larger q, there is a definite narrow peak at q=1. In order to use the Hipparcos data to full advantage, a more indirect approach is necessary. By modelling both the local Galaxy and the Hipparcos observations, one may put constraints also on the number of closer binaries which show up in the Hipparcos Catalog as non-linear proper motions.