EDP Sciences
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Volume 372, Number 3, June IV 2001
Page(s) 833 - 850
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010366

A&A 372, 833-850 (2001)
DOI: 10.1051/0004-6361:20010366

Kinematics of young stars

II. Galactic spiral structure
D. Fernández1, F. Figueras1, 2 and J. Torra1, 2

1  Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain
2  IEEC (Institut d'Estudis Espacials de Catalunya), Edif. Nexus-104, Gran Capità 2-4, 08034 Barcelona, Spain

(Received 9 February 2001 / Accepted 8 March 2001)

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 ( AOB = 13.7-13.8 km s-1 kpc-1) and a higher value for Cepheids ( ACep = 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.

Key words: Galaxy: kinematics and dynamics -- Galaxy: solar neighbourhood -- Galaxy: structure -- stars: early-type -- stars: kinematics -- stars: variables: Cepheids

Offprint request: D. Fernández, dfernand@am.ub.es

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