A SEARCH FOR SUBSTELLAR COMPANIONS TO SOUTHERN SOLAR-TYPE STARS

At the Mount John University Observatory relative radial velocities of solar-type stars have been obtained with a characteristic random erro r of 55 m s-1 using a fiber-fed echelle system and digital cross-corre lation techniques. A program of obtaining radial velocities of 29 sola r-type stars and 10 giant International Astronomical Union radial-velo city standard stars was carried out over 2.5 years with a view to the detection of low-mass companions to the dwarf stars. One dwarf star tu rned out to have a previously undiscovered stellar companion but no dw arfs showed radial-velocity variability suggestive of the presence of substellar companions, although one showed a possible variation. In co ntrast, at least half the giant or supergiant ''standard'' stars were variable in radial velocity. Four and possibly five of the giant stand ards are probably intrinsic (pulsating) red or yellow (Walker et al. 1 989) variables. Two further standards showed long-period variability s uggestive of companions of undetermined mass. The lack of brown dwarfs observed in this program is consistent with the results of other rece nt surveys. High-mass brown dwarfs appear to be rare as companions to stars and are unlikely to contribute significantly to the local mass d ensity. Low-mass brown dwarfs (or high-mass planets) seem to be rare i n orbits closer than 10 AU but could yet be found to abound in wider o rbits or in the field.