The velocity field of young stars in the solar neighbourhood

A sample of O- and B-type stars with Hipparcos astrometric data, ages compu ted from Stromgren photometry and radial velocities, has been used to chara cterize the structure, age and kinematics of the Gould Belt system. The loc al spiral structure of our galaxy is determined from this sample, and also from a sample of Hipparcos Cepheid stars. The Gould Belt, with an orientation with respect to the galactic plane of i (G) = 16-22 degrees and Omega(G) = 275-295 degrees, extends up to a distanc e of 600 pc from the Sun. Roughly the 60-65% of the O and B stars younger t han 60 Myr in the solar neighbourhood belong to this structure. Our results indicate that the kinematical behaviour of this system is complex, with an expansion motion in the solar neighbourhood (R < 300 pc). In the frame of the Lin's theory, and analysing the O and B stars further t han 600 pc and the Cepheids, we found a galactic spiral structure character ized by a 4-arm spiral pattern with the Sun located at psi(.) = 350-355 +/- 30 degrees - near the Sagittarius-Carina arm - and outside the corotation circle. The angular rotation speed of the spiral pattern was found to be Om ega(p) = 31-32 +/- 4 km s(-1) kpc(-1).