AbstractsAstronomy & Space Science

We investigate several applications of high-resolution astrometry to Galactic studies. Luyten’s NLTT, a catalog of high proper motion stars, is of limited usefulness in its original form. We therefore construct a refined version by matching to Hipparcos, Tycho-2, USNO-A and 2MASS. We improve positions to <0.1". Proper-motions are improved four-fold to 5.5 mas/yr. Further, we construct a reduced proper motion diagram allowing the classification of NLTT stars. We also produce a list of new candidate nearby white dwarfs. Using NLTT we predict nearby microlensing events to be observed with SIM, which will measure deflection of the source positions, thus providing precise (1%) mass of the lenses. We search for events using Hipparcos and NLTT (lenses), and select sources from USNO-A. NLTT lenses have inaccurate measurements leading to large uncertainties in SIM observing time. Using CCD observations of these candidates we have reduced these uncertainties. Three of the planned astrometry satellites will observe magnitude-limited samples. However, substantial additional scientific opportunities are possible if a limited number of fainter targets is included. Thus, we can increase the number of late-M dwarfs, L dwarfs, and WDs with good parallaxes by an order of magnitude, enabling determinations of local mass functions (MF). By combining SIM observations with ground-based photometry, one can solve microlensing events toward the Galactic bulge. This would allow the MF of 200 bulge objects to be measured – both luminous and non-luminous. Next we present a new method for measuring the Galactocentric distance (R₀) by solving the orbits of individual stars around it. 1-5% accuracy of R₀ is possible after 15 years of positional and radial velocity measurements. We show that combining the measurements of three stars, while constraining the center, produces considerable improvements in the R₀ determination. We propose studying the halo rotation curve by observing proper motions of faint horizontal branch stars with FAME. Halo rotation can thus be precisely (2 km/s) mapped out to 25 kpc, and halo substructure detected. Finally, we propose using SIM astrometric microlensing to measure distances and masses of enigmatic MACHO objects.