PRECISION RADIAL-VELOCITIES WITH AN IODINE ABSORPTION CELL

We have used gaseous iodine for generating reference absorption lines in stellar spectra taken at high resolution. A major advance involves the use of a fast echelle spectrograph and a 2048 X 2048 CCD which acquires the near-ultraviolet, the entire visible, and the near-infrared spectrum in a single exposure. The superimposed iodine lines provide both a highly precise wavelength scale (calibrated with a Fourier-transform spectrum) and a specification of the spectrograph PSF in situ over the entire echelle format. Test observations of three solar-type stars exhibit a velocity scatter of less than 25 m s-1 over a 1-yr duration, and only 1/5 of the available spectrum has been employed in the analysis to date. Velocity precision of 50 m s-1 can be achieved for magnitude V = 12 in 1 h exposures on a 3-m telescope. We discuss an on-going project to detect brown-dwarf and planetary companions to F-, G-, K-, and M-type main-sequence stars, designed to complement other efforts. The current velocity precision permits detection of companions with masses as low as 3 M(Jup) located up to 5 AU from the star. We also discuss the use of precision velocities in revising Cepheid distances.