The space interferometry mission astrometric grid giant star survey. I. Stellar parameters and radial velocity variability

We present results from a campaign of multiple-epoch echelle spectroscopy of relatively faint (V=9.5-13.5 mag) red giants observed as potential astrometric grid stars for the Space Interferometry Mission (SIM PlanetQuest). Data are analyzed for 775 stars selected from the Grid Giant Star Survey, spanning a wide range of effective temperatures (t _eff), gravities, and metallicities. The spectra are used to determine these stellar parameters and to monitor radial velocity (RV) variability at the 100 m s-1 level. The degree of RV variation measured for 489 stars observed two or more times is explored as a function of the inferred stellar parameters. The percentage of RV-unstable stars is found to be very high - about two-thirds of our sample. It is found that the fraction of RV-stable red giants (at the 100 m s ^-1 level) is higher among stars with T _eff ∼ 4500 K, corresponding to the calibration-independent range of infrared colors 0.59 < (J - K _s) ₀ < 0.73. A higher percentage of RV-stable stars is found if additional constraints of surface gravity and metallicity ranges, 2.3 < log g < 3.2 and -0.5 < [Fe/H] < -0.1, respectively, are applied. Selection of stars based on only photometric values of effective temperature (4300 K < T _eff < 4700 K) is a simple and effective way to increase the fraction of RV-stable stars. The optimal selection of RV-stable stars, especially in the case in which the Washington photometry is unavailable, can rely effectively on the 2MASS colors constraint 0.59 < (J - K _s) ₀ < 0.73. These results have important ramifications for the use of giant stars as astrometric references for the SIM PlanetQuest.