2000A&A...353..135A

Horizontal branch (HB) stars in the field of the Milky Way can be used as tracers for the study of early stages of the evolution of our galaxy. Since the age of individual HB stars is not known a priori, we have studied the kinematics of a sample of field HB stars measured with Hipparcos to look for signs of age and population nature. Our sample comprises 14 HBA, 2 HBB and 5 sdB/O stars. We found that the kinematics of the HBA stars is very different from that of the sdB/O stars (including those from an earlier study). The HBA stars have low orbital velocities, some are even on retrograde orbits. Their orbits have large eccentricities and in many cases reach large distances above the galactic plane. In contrast, the sdB/O stars show disk-like orbital characteristics. The few HBB stars (with T_eff>10000K) in our sample seem to have kinematics similar to that of the sdB/O stars. In order to see if there is a trend among the HB stars in their kinematics, we investigated also RR Lyrae stars measured with Hipparcos. Here we found a mixed kinematical behaviour, which was already known from previous studies. Some RR Lyrae stars have disk-like orbits (most of these being metal rich) but the majority has halo-like orbits, very similar to those of our HBA stars. Since the atmospheres of most types of HB stars do not reflect original metallicities any more the kinematics is the only aspect left to study the origin and population membership of these stars. Thus, the clear trend found in kinematics of stars along the HB, which is also a sequence in stellar mass, shows that the different kinds of field HB stars arose from stars having different origins in age and, e.g., metallicity or mass loss rate.