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 ^ { \mathrm { OB } } = 13.7-13.8 km s ^ { -1 } kpc ^ { -1 } ) and a higher value for Cepheids ( A ^ { \mathrm { 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 _ { \odot } = 284-20 \degr ) and very near the corotation circle .
The angular rotation velocity of the spiral pattern was found to be \Omega _ { \mathrm { p } } \approx 30 km s ^ { -1 } kpc ^ { -1 } .