Precision radial velocities with an iodine absorption cell

Abstract

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 2048X2048 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 one-year duration, and only 1/5 of the available spectrum has been employed in the analysis to date. Velocity precision of 50 m -1 can be achieved for magnitude V=12 in one hour 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.