The Distribution of Nearby Stars in Velocity Space Inferred from HIPPARCOS Data
Abstract
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 wellknown moving groups. While samples of earlytype stars are dominated by these maxima, also up to about a quarter of red mainsequence stars are associated with them. These moving groups are responsible for the vertex deviation measured even for samples of latetype 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_Rdirection, but not with respect to the v_phi and v_zaxes: 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.
 Publication:

The Astronomical Journal
 Pub Date:
 June 1998
 DOI:
 10.1086/300364
 arXiv:
 arXiv:astroph/9803110
 Bibcode:
 1998AJ....115.2384D
 Keywords:

 GALAXY: KINEMATICS AND DYNAMICS;
 GALAXY: STRUCTURE;
 METHODS: NUMERICAL;
 GALAXY: SOLAR NEIGHBORHOOD;
 STARS: KINEMATICS;
 Astrophysics
 EPrint:
 16 pages LaTeX (aas2pp4.sty), 6 figures, accepted by AJ