In order to perform astrometry to microarcsecond precision, SIM PlanetQuest will require a grid of astrometrically stable reference stars known as the Astrometric Grid. The goal of the Grid Giant Star Survey (GGSS) is to identify the most distant metal-poor K giants for the Astrometric Grid. (The term metal-poor refers to the metallicity or chemical composition of a star; metal-poor stars have a low abundance of heavy elements such as iron relative to lighter elements such as hydrogen). K giants will be used to make up the grid because they are numerous enough to fill an all-sky grid pattern.
The two main phases of the GGSS include (1) searching for intermediate to metal-poor giants using a specialized set of filters that allow dwarf/giant star separation, and (2) verifying the luminosity class (size and brightness) of the giant candidates, obtaining metallicities, and measuring rough radial velocities.
Majewski and collaborators have been funded by NASA to provide the GGSS. The ground-based survey identifies old, distant, metal-poor giant stars with the Swope 1-meter Telescope at the Las Campanas Observatory in Chile. Metal poor giant stars, by virtue of their luminosity, probe the Galaxy to greater distances than almost any other stellar type at the same apparent magnitude. Thus, any problems associated with astrometric jitter are reduced by the larger distances of metal poor giants. Distant (greater than 5 kiloparsecs) giants are also highly desirable for understanding and mapping the structure and kinematics of the Milky Way.
The GGSS is intended to fill partially the need of the SIM PlanetQuest mission, and may also be used to address a host of problems relating to Galactic structure and kinematics.