Terrestrial planets around M dwarfs via precise radial velocities. VLT+UVES observations of Barnard's star = GJ 699

We report on results from a program carried out at the VLT+UVES in search for terrestrial planets around M dwarf stars with precise radial velocities (RVs). Because of the small mass of M dwarfs planets with a few Earth masses and orbital periods of a few days to weeks can cause RV signals that are detectable with the currently achieved precision of RV surveys of a few m s^-1. As an example we present our results for Barnard's star (M = 0.16 M_solar) where we detect the RV secular acceleration as well as variability related to stellar activity. We determine upper limits to the projected mass m sin i of hypothetical planetary companions finding that we can exclude planets with m sin i > 7.5 M_Earth in circular orbits throughout the habitable zone of the star, i.e. at separations of 0.034 - 0.082 AU. Over the total range of separations accessible to our data set, i.e. 0.017 - 0.98 AU we exclude planets m sin i > 0.12 M_Jupiter. The lowest upper limit is found at an orbital period of 2.5 d (separation 0.020 AU) with m sin i = 2.9 M_Earth. We also find an anti-correlation of the RV data with an index that measure the filling-in by emission of the core of the H_α line in the sense that the measured RVs become bluer when the H_α emission component becomes stronger.