RR Lyrae stars trace the Milky Way warp

For those disc-like RRL we analyse their mean vertical height and mean vertical velocity. We show, for the first time, that RRL stars with thin disc-like kinematics trace the warp. In the anticentre direction, the RRL population reaches a minimum in mean vertical height of ~0.4kpc, with a trend systematically lower than the one found with Classical Cepheids.

The kinematical signal of the RRL warp starts at R~10kpc and, rather than resembling the Cepheid's, shows a similar trend to the Red Clump population from previous works, reaching a maximum value of ~7km/s in vertical velocity. We also obtain an estimation of the pattern speed of the RRL warp with a prograde rotation of ~13+/-2km/s/kpc, compatible with results obtained from Classical Cepheids. Finally, we also obtain a vertical velocity dispersion ~17km/s, inconsistent with the kinematics of a canonical old age (>10Gyr) disc population and, instead, favouring a population dominated by intermediate-age (3-4Gyr).

Our results indicate that the thin disc RRL stars are a dynamical intermediate-age tracer of the warp, opening a new window to study the dependency of the warp with stellar age in the Milky Way.