Precision narrow-angle astrometry of binary stars with the Navy Prototype Optical Interferometer

摘要

The Navy Prototype Optical Interferometry (NPOI) group has started an astrometric search for planets in binary star systems based on the idea of using the binary components as position references for one another and looking for deviations from Keplerian motion. Our search will complement the radial velocity (v_R) searches in three ways. We will observe stars of all spectral types; v_R searches are limited to the FGKM range, where stars exhibit narrow spectral lines. We will search for planets in relatively large orbits (more than about 4 AU) where our method is most sensitive; v_R searches are most sensitive to close-in planets. Finally, we will examine binary star systems, which with a few exceptions have been excluded from v_R surveys. Our targets are binaries with both components in the interferometric field of view, producing a periodic variation in the fringe visibility (V~2) across the (u, v) plane. Past NPOI results from closer binaries (separations in the tens of mas) show residuals of tens of microarcseconds about the best-fit orbits. The larger separations we are observing produce more V~2 oscillations across the (u, v) plane, offering the possibility of higher precision. We discuss the level of precision in test observations and the steps that will be needed to convert precision into accuracy.