Introduction

About six years ago we started an observational program with the FEROS spectrograph at the ESO La Silla, to investigate the radial-velocity (RV) variability in evolved late-type stars (Setiawan et al. 2003a). The main goals of this program were to determine whether G-K giants are RV variable, and why. Setiawan et al. (2004) showed that a large proportion of our stars are indeed RV variables, and two giants hosting extra-Solar planets were found (Setiawan et al. 2003b, 2005). Unfortunately, the accuracy obtained with FEROS at the ESO 1.5-m telescope was limited to about 20 m s-1; and this implies that we were missing a number of RV variables due to our limited accuracy. We therefore started a similar program in the northern sky with the 2.2-m telescope at Tautemburg Observatory (TLS) equipped

The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.

Figure 15.1. A comparison between estimated and measured stellar radii (in milli-arcseconds) for the giants of our S sample. The entries in the CHARM2 catalog were corrected for limb-darkening. The solid line represents the one-to-one relation. The large dots indicate stars with lunar occultation or LBI measurements. The fractional difference between the two samples is 6%.

Figure 15.1. A comparison between estimated and measured stellar radii (in milli-arcseconds) for the giants of our S sample. The entries in the CHARM2 catalog were corrected for limb-darkening. The solid line represents the one-to-one relation. The large dots indicate stars with lunar occultation or LBI measurements. The fractional difference between the two samples is 6%.

with an iodine cell that guarantees accuracy to within a few m s-1. This northern sample is composed of 62 stars. The first results (Dollinger et al. 2007b, A&A in preparation) show that 91% of the stars are indeed RV variables at the improved accuracy reached in our observations. Among them, we have ahandful of exoplanet-host candidates, of which one has already been confirmed (Dollinger et al. 2007a). In order to understand how RV variability of giant stars evolves with the stellar parameters, we must be able to derive firm values of stellar parameters, such as age and mass. In particular, a reliable estimate of the mass is crucial for determining the companion mass. Unlike for main-sequence stars, it is difficult to calculate stellar masses from effective temperatures of colors. We present here the first results from this analysis.

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