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

摘要

(abridged) The velocity distribution f(v) of nearby stars is estimated, via amaximum- likelihood algorithm, from the positions and tangential velocities ofa kinematically unbiased sample of 14369 stars observed by the HIPPARCOSsatellite. f(v) shows rich structure in the radial and azimuthal motions, v_Rand v_phi, but not in the vertical velocity, v_z: there are four prominent andmany 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 25%of red main-sequence stars are associated with them. These moving groups areresponsible for the vertex deviation measured even for samples of late-typestars; they appear more frequently for ever redder samples; and as a whole theyfollow an asymmetric-drift relation, in the sense that those only present inred samples predominantly have large |v_R| and lag in v_phi w.r.t. the localstandard of rest (LSR). The question arise, how these old moving groups got ontheir eccentric orbits. A plausible mechanism, known from solar systemdynamics, which is able to manage a shift in orbit space involves locking intoan orbital resonance. Apart from these moving groups, there is a smooth background distribution,akin to Schwarzschild's ellipsoidal model, with axis ratio of about 1:0.6:0.35in v_R, v_phi, and v_z. The contours are aligned with the $v_r$ direction, butnot w.r.t. the v_phi and v_z axes: the mean v_z increases for stars rotatingfaster than the LSR. This effect can be explained by the stellar warp of theGalactic disk. If this explanation is correct, the warp's inner edge must notbe within the solar circle, while its pattern rotates with frequency of about13 km/s/kpc or more retrograde w.r.t. the stellar orbits.