Precise radial velocity measurements of G and K giants

Abstract

We present the first results of our precise radial velocity ( RV ) measurements of G and K giants. A number of stars from our list of 80 targets have been observed for 14 months using the fibre-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. This sample increases the number of giants surveyed with precise stellar radial velocity measurements at least by a factor of 10. During this period we are able to estimate the long-term accuracy of our measurement as better than 11 ${\rm m\,s}^{-1}$. We use the simultaneous Th-Ar calibration and cross-correlation technique to compute the radial velocity by applying a numerical template for K-type stars. Standard deviation σ of mean radial velocity variations between 3 ${\rm m\,s}^{-1}$ and 4 k${\rm m\,s}^{-1}$ with timescales between several days and years are measured for 21 of G and K giants which are presented in this paper. Fifteen stars show definite variability above 3 σ of our measurement uncertainties. Two stars with RV variations above 800 ${\rm m\,s}^{-1}$ are tentatively identified as new binaries. Although definitive trends between RV variations and stellar evolutionary status cannot yet be established, all the luminous cool giants of our sample seem to have significant radial velocity variations, while those stars in the giant's clump region can be either variable or constant.