Jacques Lepine (University of Sao Paulo, Brazil)

"Direct measurement of the rotation speed of the spiral arms, and of the epicycle frequency in the Galactic disk"

We present two types of results that we obtained using the large database on open clusters completed by our group. The first is the determination of the rotation speed of the spiral arms of the Galaxy. The observed velocity vector (amplitude and direction) of the clusters in the galactic plane is derived from the catalog data. By integrating the orbits of the clusters towards the past for a time equal to the age of the clusters, we determine their birthplace. The position of the birthplaces as a function of age allows us to observe the rotation of the arms. We confirm in this way that the corotation radius (where rotation of the arms equal to the rotation of the galactic material) is close to the solar orbit.
    Another type of result is the direct measurement of the epicycle frequency kappa in the galactic disk. In the epicycle approximation, the observed velocity of the clusters is the sum of the circular velocity, described by the galactic rotation curve, and of a residual velocity, which has a direction that rotates with the frequency kappa. If for some reason the clusters are formed with non-random initial perturbation velocity direction (measured for instance with respect to the direction of circular rotation), then a plot of the orientation angle of the residual velocity as a function of age reveals the epicycle frequency. The data analysis confirms that this is the case; due to the non-random initial velocities, it is possible to measure the rotation rate of the velocity vectors. The values of kappa provide constraints on the rotation velocity of the disk; in particular, V0 is found to be 225 +- 15 km/s even if the short scale (R0 = 7.5 kpc) of the Galaxy is adopted.