RR Lyrae stars trace the Milky Way warp

TL;DR

This study demonstrates that RR Lyrae stars in the Milky Way's thin disc trace the Galactic warp, revealing an intermediate-age stellar population's kinematic behavior and pattern speed, thus providing new insights into the warp's age dependence.

Contribution

First detection of the warp traced by thin disc RR Lyrae stars, linking stellar age to warp structure and dynamics in the Milky Way.

Findings

RR Lyrae stars trace the Galactic warp in the anticentre direction.

Warp pattern speed estimated at approximately 13 km/s/kpc.

Vertical velocity dispersion suggests an intermediate-age stellar population.

Abstract

The outskirts of the Milky Way disc have been known to be warped since the late 1950s. Although various stellar populations have shown an underlying warped distribution, the relation between the age of the population and the warp they trace remains an open question. Our goal in this work is to detect the presence of the warp in the RR Lyrae (RRL) population of the Galactic disc. We use a compilation of public catalogues of RRL stars, precise photometric distances ($\sim 5\%$) and Gaia DR3 proper motions to kinematically select a sample of thin disc RRL in the Galactic anticentre, where the tangential velocity best approximates the azimuthal velocity to differentiate between disc and halo. For 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 $\approx 0.4$ kpc, with a trend systematically lower than the one found with Classical Cepheids. The kinematical signal of the RRL warp starts at $R\approx 10$ kpc and, rather than resembling the Cepheid's, shows a similar trend to the Red Clump population from previous works, reaching a maximum value of $\approx 7$ km/s in vertical velocity. We also obtain an estimation of the pattern speed of the RRL warp with a prograde rotation of $\approx 13 \pm 2 $ km/s/kpc, compatible with results obtained from Cepheids. Finally, we obtain a vertical velocity dispersion $\approx 17 $km/s, inconsistent with the kinematics of a canonical old age ($> 10$ Gyr) disc population and, instead, favouring a population dominated by intermediate-age ($3-4$ Gyr). 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.