Kinematics of young stars. II. Galactic spiral structure

TL;DR

This study analyzes young star velocities to map galactic spiral structure, revealing the Sun's position near the spiral arm minimum and estimating the spiral pattern's rotation speed.

Contribution

It provides new measurements of galactic rotation parameters and confirms the Sun's proximity to the spiral arm's minimum and corotation circle.

Findings

Galactic rotation curve with Oort constant A ≈ 13.7-13.8 km/s/kpc for O and B stars.

Higher A value (14.9-16.9 km/s/kpc) for Cepheid variables.

Spiral pattern rotates at approximately 30 km/s/kpc.

Abstract

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 $^{-1}$.