Studying the populations of our Galaxy using the kinematics of sdB stars ⋆

We have analysed the kinematics of a sample of 114 hot subdwarf stars. For 2/3 of the stars, new proper motions, spectroscopic and photometric data are presented. The vast majority of the stars show a kinematic behaviour that is similar to that of Thick Disk stars. Some stars have velocities rather fitting to solar, i.e. Thin Disk, kinematics. About ∼15 objects have orbital velocities which differ considerably from those of Disk stars. These are members of the Galactic Halo. We investigated the velocity dispersions and calculated the orbits. Most stars feature orbits with disk character (eccentricity of less than 0.5), a few reach far above the Galactic plane and have very eccentric orbits (eccentricity of more than 0.7). The intermediate eccentricity range is poorly populated. This seems to indicate that the (Thick) Disk and the Halo are kinematically disjunct. Plotting a histogram of the orbit data points along z leads to the z-distance probability distribution of the star; doing this for the whole sample leads to the z-distance probability distribution of the sample. The logarithmic histogram shows two slopes, each representing the scale height of a population. The disk component has a scale height of 0.9 (±0.1) kpc, which is consistent with earlier results and is similar to that of the Thick Disk. The other slope represents a component with a scale height ∼7 kpc, a much flatter gradient than for the disk component. This shows that the vast majority of the sdBs are disk stars, but a Halo minority is present, too. The kinematic history and population membership of the sdB stars on the whole is different from that of the cooler HBA stars, which are predominantly or even exclusively Halo objects. This leads to the question, whether the Halo sdB stars are of similar origin as the HBA stars, or whether their kinematical behaviour possibly represents another origin, such as infalling stellar aggregates or inner disk events.