Preparing the PRIMA astrometric planet search: selecting suitable target and reference stars

Abstract

ESO's PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at the VLT Interferometer on Cerro Paranal in Chile is expected to be fully operational in only a few years from now. With PRIMA/VLTI, it will then be possible to perform relative astrometry with an accuracy of the order of 10 microarcseconds over angles of about 10 arcseconds. The main science driver for this astrometric capability is a systematic search for extrasolar planets around nearby stars. Target stars as well as reference stars for this astrometric planet search have to be very carefully chosen in order to make the measurements robust and effective. Most importantly, reference stars have to be astrometrically stable to only a few microarcseconds in order to provide a suitable reference for the astrometric measurements. Target stars should be located at small distances so that a possible planet would cause a detectable astrometric signal. Moreover, a suitable target star and a suitable reference star have to be found within about 10 arcseconds of each other to ensure the highest accuracy and effectiveness, which obviously requires some trade-off in the final target list. Possible strategies and preparatory observations for the assembly of a suitable target list for the astrometric planet search with PRIMA/VLTI will be discussed.