Stellar encounters with the solar system

We continue our search, based on Hipparcos data, for stars which have encou ntered or will encounter the solar system (Garcia-Sanchez et al. 1999). Hip parcos parallax and proper motion data are combined with ground-based radia l velocity measurements to obtain the trajectories of stars relative to the solar system. We have integrated all trajectories using three different mo dels of the galactic potential: a local potential model, a global potential model, and a perturbative potential model. The agreement between the model s is generally very good. The time period over which our search for close p assages is valid is about +/- 10 Myr. Based on the Hipparcos data, we find a frequency of stellar encounters within one parsec of the Sun of 2.3 +/-0. 2 per Myr. However, we also find that the Hipparcos data is observationally incomplete. By comparing the Hipparcos observations with the stellar lumin osity function for star systems within 50 pc of the Sun, we estimate that o nly about one-fifth of the stars or star systems were detected by Hipparcos . Correcting for this incompleteness, we obtain a value of 11.7 +/-1.3 stel lar encounters per Myr within one pc of the Sun. We examine the ability of two future missions, FAME and GAIA, to extend the search for past and futur e stellar encounters with the Sun.