The velocity distribution f ( \mbox { \boldmath$v$ } ) 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 . f shows rich structure in the radial and azimuthal motions , v _ { R } and v _ { \varphi } , 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 _ { \varphi } w.r.t .
the local standard of rest ( LSR ) .
The question arise , how these old moving groups got on their eccentric orbits ?
A plausible mechanism known from the solar system dynamics which 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 _ { \varphi } : \sigma _ { z } \approx 1 : 0.6 : 0.35 .
The contours are aligned with the v _ { r } direction , but not w.r.t .
the v _ { \varphi } 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 \gtrsim 13 \mbox { $ { km } { s } ^ { -1 } { kpc } ^ { -1 } $ } retrograde w.r.t .
the stellar orbits .