Local stellar kinematics from Hipparcos data

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Abstract
(shortened) From a kinematically unbiased subsample of the Hipparcos catalogue we have redetermined as a function of colour the kinematics of main-sequence stars. The stars' mean heliocentric velocity nicely follows the asymmetric drift relation, except for stars blueward of B-V=0.1. Extrapolating to zero dispersion yields for the velocity of the Sun w.r.t. the LSR in km/s: U_0=10.00+/-0.36 (radially inwards), V_0=5.23+/-0.62 (in direction of galactic rotation), and W_0=7.17+/-0.38 (vertically upwards). A plot of velocity dispersion vs. colour beautifully shows Parenago's discontinuity: the dispersion is constant for B-V>0.62 and decreases towards bluer colour. We determine the velocity-dispersion tensor sigma^2_ij as function of B-V. The mixed moments involving vertical motion are zero within the errors, while sigma^2_xy is non-zero at about (10km/s)^2 independent of colour. The resulting vertex deviations are about 20 deg for early-type stars and 10+/-4 deg for old-disc stars. The persistence of the vertex deviation to late-type stars implies that the Galactic potential is significantly non-axisymmetric at the solar radius. If spiral arms are responsible for this, they cannot be tightly wound. Except for stars bluer than B-V=0.1 the ratios of the principal velocity dispersions are 2.2 : 1.4 :1, while the absolute values increase with colour from sigma_1=20km/s at B-V=0.2 to sigma_1=38km/s at Parenago's discontinuity and beyond. These ratios imply significant heating of the disc by spiral structure and that R_0/R_d=3 to 3.5, where R_d is the scale length of the disc.

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