  The kinematics of distant OB stars perpendicular to the Galactic plane,
inferred from proper motions in the Hipparcos catalogue, are analysed and
compared to the kinematic signature that would be induced by a long-lived
Galactic warp. Previously we reported that the kinematics of the OB stars
toward the anticenter were inconsistent with the assumption of a long-lived
warp (Nature 392, 471), showing negative systematic motions as opposed to the
expected positive motions. Using a larger sample of OB stars, improved
distances, and a more accurate model for the uncertainties, we confirm our
previous result for a larger range of galactocentric radii. However, we note
that the new model for errors in the photometric distances reveal an important
bias that causes the observed systematic vertical motions to be smaller than
their true values. Using synthetic catalogues we investigate the effect of this
bias on the systematic vertical motions in conjunction with the possibility of
a smaller warp amplitude and a warp precession rate, both of which can also
lead to smaller systematic motions. Taken together these three effects can work
to produce negative observed systematic vertical motions, similar to those
detected in the data, though only with both excessively high precession rates
(-25 km/sec/kpc) and very large photometric errors (1 magnitude).

Astrophysics