Preliminary Astrometric Masses for Proposed Extrasolar Planetary Companions

As a first step in the implementation of an astrometric survey, we have reduced the Hipparcos intermediate astrometric data (IAD) for 30 stars that currently exhibit small-amplitude periodic radial velocity variations. We consider only systems with periods longer than 10 days, where the motion is attributed to the presence of an ``extrasolar planetary companion'' to the star. While our results are to be used as guidance in the selection of targets for confirmation with the Multichannel Astrometric Photometer and future space-based astrometric missions (e.g., Full-Sky Astrometric Mapping Explorer and Space Interferometry Mission), they have implications for our understanding of these companions. Our initial results from this data set indicate that true, as contrasted with minimum, companion masses range from stellar to astrometric limits that indicate substellar mass; however, only one is near the minimum mass obtained by radial velocity observations. Although our studies should be considered preliminary until additional astrometric data is available, they are suggestive of a characteristic that appears to be common to a significant fraction of these systems: small sini. This, in turn, suggests that the sample of stars that have been examined for possible planetary mass companions may be biased. Moreover, if correct, these results provide a natural explanation for the fact that the orbital properties of these ``extrasolar planets'' are statistically indistinguishable from those of binary stars.