Kinematics of young stars -: II.: Galactic spiral structure

The young star velocity field is analysed by means of a galactic model which takes into account solar motion, differential galactic rotation and spiral arm kinematics. We use two samples of Hipparcos data, one containing O- and B-type stars and another one composed of Cepheid variable stars. The robustness of our method is tested through careful kinematic simulations. Our results show a galactic rotation curve with a classical value of A Oort constant for the O and B star sample (A(OB) = 13.7-13.8 km s(-1) kpc(-1)) and a higher value for Cepheids (A(Cep) = 14.9-16.9 km s(-1) kpc(-1), depending on the cosmic distance scale chosen). The second-order term is found to be small, compatible with a zero value. The study of the residuals shows the need for a K-term up to a heliocentric distance of 4 kpc, obtaining a value K = (1-3) km s(-1) kpc(-1). The results obtained for the spiral structure from O and B stars and Cepheids show good agreement. The Sun is located relatively near the minimum of the spiral perturbation potential (psi (circle dot) = 284-20 degrees) and very near the corotation circle. The angular rotation velocity of the spiral pattern was found to be Omega (p) approximate to 30 km s(-1) kpc(-1).