The dynamically selected stellar halo of the Galaxy with Gaia and the
  tilt of the velocity ellipsoid
  We study the dynamical properties of halo stars located in the Solar
Neighbourhood. Our goal is to explore how the properties of the halo depend on
the selection criteria used for defining a sample of halo stars. Once this is
understood we proceed to measure the shape and orientation of the halo's
velocity ellipsoid and we use this information to put constraints on the
gravitational potential of the Galaxy. We use the recently released Gaia DR1
catalogue cross-matched to the RAVE dataset for our analysis. We develop a
dynamical criterion based on the distribution function of stars in various
Galactic components, using action integrals to identify halo members, and
compare this to metallicity and to kinematically selected samples. With this
new method, we find 1156 stars in the Solar Neighbourhood to be likely members
of the stellar halo. Our dynamically selected sample consists mainly of distant
giants on elongated orbits. Their metallicity distribution is rather broad,
with roughly half of the stars having [M/H] $\ge -1$ dex. The use of different
selection criteria has an important impact on the characteristics of the
velocity distributions obtained. Nonetheless, for our dynamically selected and
for the metallicity selected samples, we find the local velocity ellipsoid to
be aligned in spherical coordinates in a Galactocentric reference frame: this
suggests that the total gravitational potential is rather spherical in the
region spanned by the orbits of the halo stars in these samples.
