Radio Observations of Algol . II . Computation of Close Stellar Encounters

We present here new astrometric measurements of Algol's position made with Very Long Baseline Interferometry. Using these measurements, along with a new estimate of the node of the inner binary and other orbital elements from the literature, we compute a new proper motion, with uncertainties six times less than those of Hipparcos. We compute the trajectories of Algol (using the new proper motion) and of Gliese 710, and discuss the various sources of uncertainty. We nd that it is essential to include Galactic rotation and oscillation about the midplane in order to accurately compute not only the distance of closest encounter, but also on which side of the Sun a star will pass. Finally, we compute a minimum distance criterion for individual stellar encounters to have an eeect on the observed distribution of comets. We nd the cross section for interaction for the relatively high mass to velocity ratio of Algol is an order of magnitude greater than the cross section of the inner Oort cloud. Nonetheless we nd that both the Algol and Gliese 710 encounters are too distant to produce any extra observable comets. We also compute the encounter parameters of the nearest stellar system, Cen-Proxima Cen, which we nd to be gravitionally bound. The parameters of its closest approach are similar to those of Gliese 710, and we expect no extra observable comets to result from the encounter. We note that a reanalysis of the Hipparcos data is needed to search for other potential close encounters.