Detectability of Planets in Wide Binaries by Ground-Based Relative Astrometry with AO

We observed the 3 arc sec wide visual binary star HD 19994 twice with the VLT NACO AO instrument to measure the separation between the two stars with very high precision, in order to detect the astrometric wobble of star A due to its radial velocity planet Ab, to be measured as periodic change in the binary separation. We obtained roughly 100 short exposure images per epoch and could thereby obtain a relative astrometric precision in measuring the binary separation of below 100 micro arc sec. This is the best precision ever reached in ground-based single- aperture relative astrometry, comparable to a similar program with the HST FGS by Benedict et al. With this precision, it is possible to detect the astrometric wobble (or its upper limit) of star A due to its radial velocity planet as periodic change in the binary separation, to be observed in the next few years. With such a measurement, one can determine the mass of the spectroscopic companion, which is a planet candidate from radial velocity observations (m sin i, unknown orbit inclination i). Once such a project is shown to be successful for a known planet, one can start searching for new planets in binary stars. Such observations are also very useful for significantly improving the orbit of this and many other binaries.