Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
  We present the results of a multi-component kinematic model of a large sample
of RR Lyrae detected by $Gaia$. By imposing a four-fold symmetry and employing
$Gaia$ proper motions, we are able to infer the behaviour of the velocity
ellipsoid between $\approx3$ and $\approx30$ kpc from the centre of the Galaxy.
We detect the presence of two distinct components: a dominant non-rotating
halo-like population and a much smaller rotating disc-like population. We
demonstrate that the halo RR Lyrae can be described as a superposition of an
isotropic and radially-biased parts. The radially-biased portion of the halo is
characterised by a high orbital anisotropy $\beta\approx0.9$ and contributes
between 50\% and 80\% of the halo RR Lyrae at $5<R$(kpc)$<25$. In line with
previous studies, we interpret this high-$\beta$ component as the debris cloud
of the ancient massive merger also known as the $Gaia$ Sausage (GS) whose
orbital extrema we constrain. The lightcurve properties of the RR Lyrae support
the kinematic decomposition: the GS stars are more metal-rich and boast higher
fractions of Oosterhoff Type 1 and high amplitude short period (HASP) variables
compared to the isotropic halo component. The metallicity/HASP maps reveal that
the inner 10 kpc of the halo is likely inhabited by the RR Lyrae born in-situ.
The mean azimuthal speed and the velocity dispersion of the disc RR Lyrae out
to $R\approx30$ kpc are consistent with the behaviour of a young and metal-rich
thin disc stellar population.
