The kinematic signature of the Galactic warp in Gaia DR1 - I. The
  Hipparcos subsample
  The mechanism responsible for the warp of our Galaxy, as well as its
dynamical nature, continues to remain unknown. With the advent of high
precision astrometry, new horizons have been opened for detecting the
kinematics associated with the warp and constraining possible warp formation
scenarios for the Milky Way. The aim of this contribution is to establish
whether the first Gaia data release (DR1) shows significant evidence of the
kinematic signature expected from a long-lived Galactic warp in the kinematics
of distant OB stars. As the first paper in a series, we present our approach
for analyzing the proper motions and apply it to the sub-sample of Hipparcos
stars. We select a sample of 989 distant spectroscopically-identified OB stars
from the New Reduction of Hipparcos, of which 758 are also in Gaia DR1,
covering distances from 0.5 to 3 kpc from the Sun. We develop a model of the
spatial distribution and kinematics of the OB stars from which we produce the
probability distribution functions of the proper motions, with and without the
systematic motions expected from a long-lived warp. A likelihood analysis is
used to compare the expectations of the models with the observed proper motions
from both Hipparcos and Gaia DR1. We find that the proper motions of the nearby
OB stars are consistent with the signature of a warp, while those of the more
distant stars (parallax<1 mas) are not. The kinematics of our sample of young
OB stars suggests that systematic vertical motions in the disk cannot be
explained by a simple model of a stable long-lived warp. The Galactic warp may
either be a transient feature, or additional phenomena are acting on the
gaseous component of the Milky Way, causing systematic vertical motions that
are masking the expected warp signal. A larger and deeper sample of stars with
Gaia astrometry will be needed to constrain the dynamical nature of the
Galactic warp.
