Analysis of the Hipparcos observations of the extrasolar planets and the brown dwarf candidates

We analyzed the Hipparcos astrometric observations of 47 stars that were di scovered to harbor giant planets and 14 stars with brown dwarf secondary ca ndidates. The Hipparcos measurements were used together with the correspond ing stellar radial velocity data to derive an astrometric orbit for each sy stem. To find out the significance of the derived astrometric orbits, we ap plied a "permutation" technique by which we analyzed the permuted Hipparcos data to get false orbits. The size distribution of these false orbits indi cated the range of possibly random, false orbits that could be derived from the true data. These tests could not find any astrometric orbit of the pla net candidates with significance higher than 99%, suggesting that most if n ot all orbits are not real. Instead, we used the Hipparcos data to set uppe r limits on the masses of the planet candidates. The lowest derived upper l imit is that of 47 UMa-0.014 M., which confirms the planetary nature of its unseen companion. For 13 other planet candidates, the upper limits exclude the stellar nature of their companions, although brown dwarf secondaries a re still an option. These negate the idea that all or most of the extrasola r planets are disguised stellar secondaries. Of the 14 brown dwarf candidat es, our analysis reproduced the results of Halbwachs et al., who derived si gnificant astrometric orbits for six systems that imply secondaries with st ellar masses. We show that another star, HD 164427, which was discovered on ly very recently, also has a secondary with stellar mass. Our findings supp ort Halbwachs et al.'s conclusion about the possible existence of the "brow n dwarf desert" that separates the planets and the stellar secondaries.