A Strategy for Identifying the Grid Stars for theSpace Interferometry Mission

Abstract

We present a strategy to identify several thousand stars that are astrometrically stable at the micro-arcsecond level for use in the SIM (Space Interferometry Mission) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into account we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star due to their intrinsic brightness. We show that it is possible to identify suitable candidate grid K giants from existing astrometric catalogs. However, double stars have to be eliminated from these candidate grid samples, since they generally produce much larger astrometric jitter than tolerable for the grid. The most efficient way to achieve this is probably by means of a radial velocity survey. To demonstrate the feasibility of this approach, we repeatedly measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos K giants with precisions of 5-8 m s −1 . The distribution of the intrinsic radial velocity variations for the bona-fide single K giants shows a maximum around 20 m s −1 , which is small enough not to severely affect the identification of stellar companions around other K giants. We use the results of our observations as input parameters for Monte-Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with periods mostly larger than 200 years, but only 3.6% of all grid stars would display astrometric jitter larger than 1 µas. This contamination level is probably tolerable.

The HARPS search for southern extra-solar planets

“…In order to better reflect the actual velocity jitter and to get a reduced χ 2 closer to 1, we quadratically added 25 m/s to the individual errors. This jitter amplitude is common for a K giant with B − V = 1.35, a range where it may vary from 20 to 100 m/s (Frink et al 2001). The residuals show a slight negative slope, indicating a possible distant companion.…”

Analytic Study of Grid Star and Reference Star Selection for theSpace Interferometry Mission

“…In the next two sections on astrometric contamination (°4) and implications (°5), I treat these two classes in parallel. In°6, I test the analytic formulae derived here by "" predicting ÏÏ the results of a Monte Carlo simulation by Frink et al (2001). I Ðnd excellent agreement.…”

“…For example, reducing from 60 to 20 m s~1 would decrease the p RV number of stellar contaminants by * ln G(K) D 36%. It would not be possible to go below 20 m s~1, since this is the typical scale of photospheric Ñuctuations of K giants (Frink et al 2001). However, if need be, one could achieve the same e †ect with multiple measurements (assuming, as is almost certainly the case, that these Ñuctuations overwhelmingly have power on short timescales, so that they do not couple to the acceleration measurements).…”

“…Comparison with the Monte Carlo simulation of Frink et al (2001) permits a direct check of the foregoing calculation, but Ðrst it is necessary to translate their parameterization into the one used here. They consider an RV survey with N \ 2 epochs separated by yr, each with T RV \ 5 p RV \ J2 m s~1 (including measurement errors and the intrin-] 20 sic instability of stellar atmospheres).…”

“…This problem has to date been addressed primarily by Monte Carlo simulations (Frink et al 2001 ;C. S. Jacobs 2000, private communication ;D.…”