A Search for Substellar Companions to Solar-type Stars

Relative radial velocities with a mean external error of 13 m/s rms have been obtained for 12 late-type dwarfs and four subgiants over the past six years. Two stars, Chi1 Ori A and Gamma Cep, show large velocity variations probably due to stellar companions. In contrast, the remaining 14 stars are virtually constant in velocity, showing no changes larger than about 50 m/s. No obvious variations due to effects other than center-of-mass motion, including changes correlated with chromospheric activity, are observed. Seven stars show small, but statistically significant, long-term trends in the relative velocities. These cannot be due to about 10-80 Jupiter mass brown dwarfs in orbits with P less than about 50 yr, since these would have been previously detected by conventional astrometry; companions of about 1-9 Jupiter masses are inferred. Since relatively massive brown dwarfs are rare or nonexistent, at least as companions to normal stars, these low-mass objects could represent the tip of the planetary mass spectrum. Observations are continuing to confirm these variations, and to determine periods.