The Distribution of Nearby Stars in Velocity Space

From the tangential velocities of stars all over the sky, one can, in a statistical way, infer their 3D velocity distribution. An application to Hipparcos data reveals rich structure in the planar stellar motions: there are several moving groups, increasing in number with stellar type but decreasing in importance. A distinct group of outward moving stars with low rotation velocities might be associated with the Galactic bar: growing a bar in a smooth model for the Galactic disk results in such a group, provided the Sun is outside the outer Lindblad resonance (OLR) and the orientation angle with the bar is in the first quadrant. The vertical motions show less structure, but the mean v̄z increases for large rotational velocities, which can be nicely explained by a warp of the outer stellar disk. 1. Inferring f(v) from Hipparcos Data ESA’s Hipparcos mission measured the positions, parallaxes, and proper motions of many nearby stars. The radial velocity (RV) of most of these stars is unknown (using only stars with known RVs gives a sample strongly biased towards high-v stars [Binney et al., 1997]), and we cannot evaluate their space motions v, but only the tangential velocities vt. However, using stars from all over the sky, we can infer their distribution f(v) in a statistical way, for instance, by maximizing the log-likelihood