SCYON Abstract

Received on October 11 2006

Statistics of initial velocities of open clusters

Authors	J.R.D. Lépine (¹), W.S. Dias (²), and Yu. Mishurov (³)
Affiliation	(¹) IAG, Universidade de Sao Paulo, Brazil (²) UNIFEI - Universidade Federal de Itajuba, MG, Brazil (³) Rostov State University, Russia
Accepted by	Conference Proceedings
Contact	jacques@iagusp.usp.br
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Abstract

We made use of our large database of galactic clusters, which contains distances, proper motions and radial velocities, to determine the initial velocities of these objects (direction in the galactic plane, and amplitude). In a previous work (Dias et al. 2005) we showed that the birth of open clusters occurs in the spiral arms. By integrating backwards the galactic orbits of the clusters for a time equal to their age, we retrieved the birthplaces as a function of time and we determined the rotation speed of the spiral pattern. Now we use the same method to retrieve the initial velocities, and we measure the angle of the initial velocity perturbation with respect to the direction of circular motion. We find that the clusters are not born with random velocities, but with velocities that are organized in a few preferential directions with respect to the spiral arms. The existence of preferential initial directions allows us to directly observe the epicycle frequency, by plotting the orientation angle of the residual velocity (after the removal of the normal circular velocity) as a function of age. Our results show that a preferential direction of birth velocity can survive for times longer than 100 Myr. This can be explained by star-formation in spiral shock waves, but excludes some other star formation mechanisms such as star formation induced by supernovae.