Astrometric Grid and Reference Stars for the Space Interferometry Mission

Sabine Frink

poster presented at the AAS 198 in Pasadena, June 3-7, 2001


The Space Interferometry Mission (SIM) is designed to perform astrometry at the 4 µas level in its wide-angle mode and at the 1 µas level in its narrow-angle mode. This unprecedented accuracy strongly depends on the astrometric stability of grid and reference stars. The grid consists of several thousand stars distributed uniformly over the sky and allows to tie the individual measurements into a global reference frame (wide-angle mode). In addition, reference stars located close to the targets are needed to obtain the highest astrometric precision (narrow-angle mode).

It has been shown (Patterson et al. 1999, Frink et al. 2001) that K giants are the best type of stars for use as grid and reference objects. However, double and multiple stars with periods smaller than about 300 years have to be eliminated, since their photocenter motion would exceed the tolerable amount for astrometric stability. This could be achieved with an extensive radial velocity survey (Frink et al. 2001, Gould 2001).

Here we focus on the availability of suitable K giant input samples from astrometric catalogs as well as on the constraints regarding photometric stability in unidentified wide binary systems.

ps-file of scaled-down poster (339 kb)