Direct determination of the spiral pattern rotation speed of the Galaxy

The rotation velocity of the spiral pattern of the Galaxy is determined by direct observation of the birthplaces of open clusters of stars in the galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, like the existence of a given number of spiral arms, or the presence of a bar in the central regions. This study became possible due to the recent completion of a large database on open clusters by our group. The birthplaces of the clusters are determined by two methods, one that assumes that the orbits are circular, and the other by integrating the orbits in the Galactic potential for a time equal to the age of the clusters. We selected in the database a sample of 212 clusters for which proper motions, radial velocities, distances and ages are available, or of 612 clusters that have ages and distances available. We tested different assumptions concerning the rotation curve and the radius R0 of the solar orbit. Our results confirm that a dominant fraction of the open clusters are formed in spiral arms, and that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves. We find that the corotation radius Rc is close to the solar galactic orbit (Rc/R0 = 1.08 ± 0.08). This proximity has many potentially interesting consequences, like a better preservation of life on the Earth, and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the galactic disk. Subject headings: Galaxy: spiral arms: Galaxy corotation