Parallaxes, Proper Motions and Low-mass Companion Detection Limits for Proxima Centauri and Barnard's Star

We report on astrometric efforts to detect planetary-mass companions associated with two M dwarf stars, Proxima Centauri and Barnard's Star (0.10 and 0.12 solar masses). Our astrometer is a white-light interferometer (Fine Guidance Sensor 3) aboard the Hubble Space Telescope. Recent re-calibrations, including a final Lateral Color Calibration (FGS 3 contains refractive optics) have improved the precision of parallax and proper motion modeling, and, hence, unseen companion detection limits. With three additional Proxima Centauri observation sets since March 1997, we have companion limits below Saturn mass for periods P > 400 days. Detection limits for shorter period companions are higher, approaching one Jupiter mass at P= 40 days. For Barnard's Star we reach a detection limit of about 0.4 Jupiter for P = 400 days, confirming and somewhat extending the limits obtained by Gatewood (1995, Ap&SS, 223, 91). We briefly discuss our null results in the context of planet formation theories and other (successful) planet searches.