The population membership of sdB stars: a kinematic analysis

We analysed the kinematics of a sample of 110 sdB stars, presenting new data for about 213 of the sample. The vast majority of the stars shows a kinematic behaviour similar to that of thick disk stars. About 15 objects have orbital velocities differing substantially from those of the rest. Some have solar (i.e. thin disk) velocities. The dispersion of the orbital velocities of the full sample is somewhat larger than expected for a thick disk alone. Therefore the sdB stars belong to both the thin and thick disk, as well as to a halo component. In addition to a study of the kinematics, we calculated the orbits using a galactic potential model. While most stars have disk type orbits, a few venture far above the galactic plane. Most orbits have eccentricities of less than 0.5, a few of more than 0.7, with the region in between somewhat underpopulated. This indicates that the (thick) disk and the halo are kinematically disjunct. The statistics of the z-distance at given time intervals in the orbits of the stars leads to the z-probability distribution of the sample. For the disk component we so find a scale height of similar to1 kpc, which is consistent with the result of an earlier study and also similar to other results for the scale height of the thick disk. The other component has a much larger scale height. Evidently, most sdB stars of our sample belong to the disk, but a halo minority exists. Therefore in general, the kinematic history of the sdBs is different from that of the cooler HBA stars, which are mainly or even exclusively halo objects.