SEARCHING FOR FAINT COMPANIONS TO NEARBY STARS WITH THE HUBBLE-SPACE-TELESCOPE

A search for faint companions (FCs) to selected stars within 5 pc of t he Sun using the Hubble Space Telescope's Planetary Camera (PC) has be en initiated. To assess the PC's ability to detect FCs, we have constr ucted both model and laboratory-simulated images and compared them to actual PC images. We find that the PC's point-spread function (PSF) is 3-4 times brighter over the angular range 2 ''-5 '' than the PSF expe cted for a perfect optical system. Azimuthal variations of the PC's PS F are 10-20 times larger than expected for a perfect PSF. These variat ions suggest that light is scattered nonuniformly from the surface of the detector. Because the anomalies in the PC's PSF cannot be precisel y simulated, subtracting a reference PSF from the PC image is problema tic. We have developed a computer algorithm that identifies local brig htness anomalies within the PSF as potential FCs. We find that this se arch algorithm will successfully locate FCs anywhere within the circum stellar field provided that the average pixel signal from the FC is at least 10 sigma above the local background. This detection limit sugge sts that a comprehensive search for extrasolar Jovian planets with the PC is impractical. However, the PC is useful for detecting other type s of substellar objects, With a stellar signal of 10(9) e(-), for exam ple, we may detect brown dwarfs as faint as M(l)=16.7 separated by 1 ' ' from a Cen A.