The Kinematics and Nature of Gould ' S Belt � a 30 Myr Old

For a sample of 2440 non-supergiant Hipparcos stars belonging to Strr omgren's `early group' luminosities, temperatures, ages and distances have been determined. 241 stars younger than 30 Myr with well determined ages and space velocities fall within the local attened system, inclined 20 o to the galactic plane, called Gould's Belt. The system is dominated by a number of nearby prominent associations within a distance of 700 pc, foremost the Scorpius-Centaurus and Orion associations. We derive galactic rotation parameters for young stars of ages less than 30 Myr situated outside the Gould Belt region. This gives a at rotation curve with a circular angular velocity at the Sun of c = 25:3 1:5 km s ?1 kpc ?1. The kinematics of Gould's Belt diiers signiicantly from that of stars outside the system. Besides a slight outwards motion of a few km s ?1 it rotates in the same direction as the galactic rotation and expands, giving apparent values for the galactic rotation parameters of B = ?21 and K = +12 km s ?1 kpc ?1. It is suggested that this system was born about 30 ? 40 Myr ago, possibly in a spiral arm, with an angular momentum too large to keep the system gravitation-ally bound. The rotation may explain the atness of this inclined system.