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 (Teff), gravities and metallicities. The spectra are used to determine these stellar parameters and to monitor radial velocity (RV) variability at the 100 m/s 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 radial velocity unstable stars is found to be very high -- about 2/3 of our sample. It is found that the fraction of RV-stable red giants (at the 100 m/s level) is higher among stars with Teff \sim 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 the 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 < Teff < 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 when the Washington photometry is unavailable, can rely effectively on 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. 
Dmitry Bizyaev (1,2), Verne V. Smith (1,3), Jose Arenas (4), Doug   Geisler (4), Steven R. Majewski (5), Richard J. Patterso...