2009MNRAS.396..553D

We use accurate absolute proper motions and Two-Micron All-Sky Survey K_s-band apparent magnitudes for 364 Galactic RR Lyrae variables to determine the kinematical parameters of the Galactic RR Lyrae population and constrain the zero-point of the K_s-band period-luminosity relation for these stars via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be [U₀(Halo), V₀(Halo), W₀(Halo)] = (-12±10, -217±9, -6±6)km/s and [U₀(Disc), V₀(Disc), W₀(Disc)] = (-15±7, -44±7, -25±5)km/s, respectively, and the corresponding components of the velocity-dispersion ellipsoids, [σV_R(Halo), σV_θ(Halo), σW(Halo)] = (167±9, 86±6, 78±5)km/s and [σV_R(Disc), σV_θ(Disc), σW(Disc)] = (55±7, 44±6, 30±4)km/s, respectively. The fraction of thick-disc stars is estimated at 0.25±0.03. The corrected infrared period-luminosity relation is, implying a Large Magellanic Cloud (LMC) distance modulus of 18.27±0.08 and a solar Galactocentric distance of 7.58±0.40 kpc. Our results suggest no or slightly prograde rotation for the population of halo RR Lyraes in the Milky Way.