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The Hubble Space Telescope Astrometry Science Team has collected over 40 sets of observations of a star field containing Proxima Cen. Our primary goal is to determine an upper limit on planetary companions. Monte Carlo simulations suggest that an 0.001 arcsec amplitude perturbation would be detected in these data. Assuming M(PROXIMA)=0.1M(Sun) gives detection limits: P(days) M (Jupiter = 1) 200 0.45 150 0.5 100 0.7 50 1.1 From an analysis of our reference star astrometry we determine that our per observation astrometric precision is at or below 0.002 arcsec per axis for stars located in the center of FGS 3. We determine the proper motion and parallax of Proxima Cen with an internal precision of 0.0004 arcsec, relative to a reference frame of 8 stars with average V = 15. Subjecting the Proxima Cen position residuals to periodogram analysis, we find a peak near 77 days for both the x and y position residuals. There is a 25% probability that this is a false-positive detection. The 77 day variation could be due to a planet with 0.8 the mass of Jupiter, separated from Proxima Cen by 0.17 AU. Given the very low luminosity of Proxima Cen, a gas giant planet could survive at this distance. FGS 3 is a photometer. We found Proxima Cen to be a variable star (Benedict et al. 1993, PASP, 105, 487). Additional observations have eliminated the original 40 day variation in favor of an 80 day period. This is likely due to rotational modulation of bright or dark star spots. Note the coincidence of the astrometric with the photometric period. Since FGS 3 obtains positions from light measuring devices (PMT), we explore the possibility that photometric and positional variations with time occur within FGS 3 or HST. Thus far we have eliminated many instrumental effects, and are continuing our analysis until we exhaust all possible. The perturbation has motivated in-progress ground-based photometric and radial velocity monitoring. Radial velocity variations could be as large as +/- 200 m/s. The existence of this planet could be confirmed or refuted by Fall 1994.
