Stellar encounters with the solar system

We continue our search, based on Hipparcos data, for stars which have encountered or will encounter the solar system (Garcia-Sanchez et al. 1999). Hipparcos parallax and proper motion data are combined with ground-based radial velocity measurements to obtain the trajectories of stars relative to the solar system. We have integrated all trajectories using three different models of the galactic potential: a local potential model, a global potential model, and a perturbative potential model. The agreement between the models is generally very good. The time period over which our search for close passages 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 luminosity function for star systems within 50 pc of the Sun, we estimate that only 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 stellar 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 future stellar encounters with the Sun.