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We present results from a campaign of multiple epoch echelle spectroscopy ofrelatively faint (V = 9.5-13.5 mag) red giants observed as potentialastrometric grid stars for the Space Interferometry Mission (SIM PlanetQuest).Data are analyzed for 775 stars selected from the Grid Giant Star Surveyspanning a wide range of effective temperatures (Teff), gravities andmetallicities. The spectra are used to determine these stellar parameters andto monitor radial velocity (RV) variability at the 100 m/s level. The degree ofRV variation measured for 489 stars observed two or more times is explored as afunction of the inferred stellar parameters. The percentage of radial velocityunstable stars is found to be very high -- about 2/3 of our sample. It is foundthat the fraction of RV-stable red giants (at the 100 m/s level) is higheramong stars with Teff \sim 4500 K, corresponding to the calibration-independentrange of infrared colors 0.59 < (J-K_s)_0 < 0.73. A higher percentage ofRV-stable stars is found if the additional constraints of surface gravity andmetallicity ranges 2.3< log g < 3.2 and -0.5 < [Fe/H] < -0.1, respectively, areapplied. Selection of stars based on only photometric values of effectivetemperature (4300 K < Teff < 4700 K) is a simple and effective way to increasethe fraction of RV-stable stars. The optimal selection of RV-stable stars,especially in the case when the Washington photometry is unavailable, can relyeffectively on 2MASS colors constraint 0.59 < (J-K_s)_0 < 0.73. These resultshave important ramifications for the use of giant stars as astrometricreferences for the SIM PlanetQuest.Comment: Astronomical Journal, in press, 22 pages, 11 Postscript figures, uses  aastex.cl

TheSpace Interferometry MissionAstrometric Grid Giant Star Survey. I. Stellar Parameters and Radial Velocity Variability