The Distribution of Nearby Stars in Velocity Space Inferred from HIPPARCOS Data

The velocity distribution f(v) of nearby stars is estimated, via a maximum likelihood algorithm, from the positions and tangential velocities of a kinematically unbiased sample of 14,369 stars observed by the Hipparcos satellite. The distribution f shows rich structure in the radial and azimuthal motions, v_R and v_phi, but not in the vertical velocity, v_z: there are four prominent and many smaller maxima, many of which correspond to well-known moving groups. While samples of early-type stars are dominated by these maxima, also up to about a quarter of red main-sequence stars are associated with them. These moving groups are responsible for the vertex deviation measured even for samples of late-type stars; they appear more frequently for ever redder samples, and as a whole they follow an asymmetric drift relation, in the sense that those only present in red samples predominantly have large |v_R| and lag in v_phi with respect to the local standard of rest (LSR). The question arises, how did these old moving groups get on their eccentric orbits? A plausible mechanism known from solar system dynamics that is able to manage a shift in orbit space is sketched. This mechanism involves locking into an orbital resonance; in this respect is intriguing that Oort's constants, as derived from Hipparcos data, imply a frequency ratio between azimuthal and radial motion of exactly Omega:kappa = 3:4. Apart from these moving groups, there is a smooth background distribution, akin to Schwarzschild's ellipsoidal model, with axis ratios sigma_R:sigma_phi:sigma_z ~ 1:0.6:0.35. The contours are aligned with the v_R-direction, but not with respect to the v_phi- and v_z-axes: the mean v_z increases for stars rotating faster than the LSR. This effect can be explained by the stellar warp of the Galactic disk. If this explanation is correct, the warp's inner edge must not be within the solar circle, while its pattern rotates with frequency >~13 km s^-1 kpc^-1 retrograde with respect to the stellar orbits.