A. Siebert, B. Famaey, J. Binney, B. Burnett, C. Faure, I. Minchev, M. E. K. Williams, O. Bienayme, J. Bland-Hawthorn, C. Boeche, B. K. Gibson, E. K. Grebel, A. Helmi, A. Just, U. Munari, J. F. Navarro, Q. A. Parker, W. A. Reid, G. Seabroke, A. Siviero, M. Steinmetz, T. Zwitter
Using the RAVE survey, we recently brought to light a gradient in the mean galactocentric radial velocity of stars in the extended solar neighbourhood. This gradient likely originates from non-axisymmetric perturbations of the potential, among which a perturbation by spiral arms is a possible explanation. Here, we apply the traditional density wave theory and analytically model the radial component of the two-dimensional velocity field. Provided that the radial velocity gradient is caused by relatively long-lived spiral arms that can affect stars substantially above the plane, this analytic model provides new independent estimates for the parameters of the Milky Way spiral structure. Our analysis favours a two-armed perturbation with the Sun close to the inner ultra-harmonic 4:1 resonance, with a pattern speed \Omega_p=18.6^{+0.3}_{-0.2} km/s/kpc and a small amplitude A=0.55 \pm 0.02% of the background potential (14% of the background density). This model can serve as a basis for numerical simulations in three dimensions, additionally including a possible influence of the galactic bar and/or other non-axisymmetric modes.
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http://arxiv.org/abs/1207.0363
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