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Space Telescope Science Institute
Radial Velocity Searches for Low-Mass Companions to Stars

David W. Latham
Harvard-Smithsonian Center for Astrophysics
Cambridge MA

The frequency of substellar companions orbiting solar-type stars has proven to be rather low, on the order of a few percent for periods shorter than a few years and masses larger than one Jupiter. This can explain the failure of a Canadian team to detect any planets with their pioneering HF absorption-cell technique for measuring highly precise velocities with the CFHT coude spectrograph: their sample included only 16 good targets, and they were unlucky. The first interesting low-mass companion to a solar-type star, HD 114762, was discovered instead at a much lower velocity precision, as part of a large survey. Such surveys with the CORAVEL spectrometers and CfA Digital Speedometers have found only a limited number of companions with minimum masses in the range 10 to 80 Jupiters, suggesting that brown dwarf companions are rather rare. Large surveys with much better precision (e.g. with ELODIE, the Hamilton echelle, the McDonald coude, the LPL Fabry-Perot instrument, and the AFOE) have pushed the detection limits well below one Jupiter, but here also the frequency of detected companions is low. Nevertheless, enough companions have been discovered recently to allow a tantalizing glimpse of patterns in the orbital parameters, such as eccentricity and minimum mass. With new instruments such as HIRES on Keck, CORALEE, and the Hobby-Eberley telescope, there is the promise of excellent progress over the next few years in the identification of dozens of additional systems. This may clarify the mechanisms involved in the formation and evolution of low-mass companions.