Kinematical trends among the field horizontal branch stars

Abstract

Field Horizontal branch (HB) stars 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. We found that the kinematics of the HBA stars is very different from that of the sdB/O stars. The HBAs have low, sometimes negative, orbital velocities and feature orbits with large eccentricities, often venturing far above the galactic plane. In contrast, the sdB/O stars show disk-like orbital characteristics. The HBB stars (T_eff > 10,000 K) show kinematics similar to that of the sdB/Os. 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 and found a mixed kinematical behaviour, already known from previous studies. 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 their origin and population membership. 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.