We investigate the choice of stellar population for use as the astrometric grid for the Space Interferometry Mission (SIM). SIM depends on the astrometric stability of about 2000 stars, the so-called grid, against which the science measures are referenced. Low-metallicity and thus relatively high luminosity K giants are shown to be the population of choice, when available. The alternative, nearby G dwarfs, is shown to be susceptible to unmodeled motions induced by gas-giant planetary companions, should there be a significant population of such companions. Radial velocity filtering is quite efficient in selecting grid members from the K giants with yields exceeding 50% if filtering at 30 m s-1 (1 σ) is available. However, if the binary fraction of the G dwarfs approaches 100% as some studies suggest, the yield of stable systems would be in the range of 15% at best (with 10 m s-1 filtering). Use of the initial SIM measurement as a final filter is shown not to be critical in either case, although it could improve the yield of stable grid members. For a grid composed of weak-lined K giants, the residual contamination by large unmodeled motions will amount to about 3% (and rises to about 10% if a 60 m s-1 radial velocity criterion is used). The selective introduction of quadratic terms in the proper motion solutions during the postmission phase of data reduction can reduce contamination to a remarkable 1% or better in either case. Analytic estimates based on circular orbits are developed that show how these results come about.
Publications of the Astronomical Society of the Pacific
- Pub Date:
- September 2002
- Stars: Binaries: General;
- space vehicles;
- Techniques: Interferometric;
- 42 pages including 13 eps figures. To be published Sept 2002 in PASP