Abstract. We continue our search, based on Hipparcos data, for stars which have encountered or will encounter the solar system (García-Sánchez 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.

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The structure of AGN accretion disks on sub-parsec scales can be probed through free-free absorption of synchrotron emission from the base of symmetric radio jets. For objects in which both jet and counterjet are detectable with VLBI, the accretion disk will cover part of the counterjet and produce diminished brightness whose angular size and depth as a function of frequency can reveal the radial distribution of free electrons in the disk. The nearby (41 Mpc, independent of H 0 ) FR-I radio galaxy NGC 4261 contains a pair of symmetric kpc-scale jets. On parsec scales, radio emission from the nucleus is strong enough for detailed -2imaging with VLBI. We present new VLBA observations of NGC 4261 at 22 and 43 GHz, which we combine with previous observations at 1.6 and 8.4 GHz to map absorption caused by an inner accretion disk. The relative closeness of NGC 4261 combined with the high angular resolution provided by the VLBA at 43 GHz gives us a very high linear resolution, approximately 2 × 10 −2 pc ≈ 4000 AU ≈ 400 Schwarzchild radii for a 5 × 10 8 M ⊙ black hole. The jets appear more symmetric at 1.6 GHz because of the low angular resolution available. The jets are also more symmetric at 22 and 43 GHz, presumably because the optical depth of free-free absorption is small at high frequencies. At 8.4 GHz neither confusion effect is dominant and absorption of counterjet emission by the presumed disk is detectable. We find that the orientation of the radio jet axis is the same on pc and kpc scales, indicating that the spin axis of the inner accretion disk and black hole has remained unchanged for at least 10 6 (and more likely > 10 7 ) years. This suggests that a single merger event may be responsible for the supply of gas in the nucleus of NGC 4261. The jet opening angle is between 0.3 • and 20 • during the first 0.2 pc of the jet, and must be < 5 • during the first 0.8 pc. Assuming that the accretion disk is geometrically and optically thin and composed of a uniform 10 4 K plasma, the average electron density in the inner 0.1 pc of the disk is 10 3 − 10 8 cm −3 . The mass of ionized gas in the inner pc of the disk is 10 1 −10 3 M ⊙ , sufficient to power the radio source for ∼ 10 4 −10 6 years. Equating thermal gas pressure and magnetic field strength gives a disk magnetic field of ∼ 10 −4 − 10 −2 gauss at 0.1 pc. We include an appendix containing expressions for a simple, optically thin, gas pressure dominated accretion disk model which may be applicable to other galaxies in addition to NGC 4261.