The Impact of Tides on Star Cluster Formation
Résumé
The homogeneity of metal abundances in clusters suggests that the epoch of star formation must have been {short} in order to avoid strong gradients in the stellar population, leading to systems with little velocity dispersion at formation. When {spherical clusters} undergo violent relaxation a good fraction of stars describe nearly radial orbits. This situation likely gives rise to triaxial instability (see e.g. Merritt 1999). Aspherical collapse also results in triaxial equilibria. However a net torque may now be exerted on individual clusters by the host galaxy's tide during radial infall, and induce streaming motion or rotation. We investigate analytically the collapse of uniform-density spheroids to which we apply model galactic tides. We find that sub-virial clusters non-rotating at birth may yet show appreciable rotation in equilibrium.