The kinematic signature of the Galactic warp in Gaia DR1 I. The Hipparcos sub-sample

Context. The mechanism responsible for the warp of our Galaxy continues to remain unknown, as well as its dynamical nature. 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. Aims. The aim is to establish whether the first Gaia data release (DR1) already 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 here present our approach for analyzing the proper motions and apply it to the sub-sample of Hipparcos stars. Methods. We select a sample of distant spectroscopically-identified OB stars from the New Reduction of Hipparcos (van Leeuwen 2007), and a subset that are also in the first Gaia data release (DR1). We develop a model of the spatial distribution and kinematics of the OB stars from which we produce synthetic catalogues with the same error properties as our two samples, and compare the proper motion distributions produced including (or not) the systematic motions expected from a long-lived warp with the observed (pre-Gaia) Hipparcos proper motions of the OB stars and with the subset of Hipparcos OB stars in Gaia DR1. Results. We find that neither the Gaia nor Hipparcos proper motions of the OB stars clearly support the presence of warp-induced vertical motions, notwithstanding the significant improvement in the precisions of the proper motions for the Hipparcos stars found in Gaia DR1. Indeed, the observed dispersion of proper motions is dominated by the intrinsic velocity dispersions in this sample, whose main limitation is its size. Conclusions. A larger and deeper sample of stars with Gaia astrometry will be needed to unambiguously reveal and constrain the dynamical nature of the Galactic warp.This is the submitted version of the paper. Comments to poggio@oato.inaf.it and drimmel@oato.inaf.i