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 ($A^{\mathrm{OB}} =$ 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 $^{-1}$.