RR Lyrae stars trace the Milky Way warp

¹ Departamento de Astronomía, Instituto de Física, Universidad de la República, Iguá 4225, CP 11400 Montevideo, Uruguay
² National Astronomical Observatory of Japan, Mitaka-shi, Tokyo 181-8588, Japan

^★ Corresponding author: mauro.cabrera@pedeciba.edu.uy

Received: 24 October 2024
Accepted: 22 July 2025

Abstract

Context. 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. Understanding this relation may shed light on the origin of the warp, which remains a puzzle to be solved.

Aims. Our goal in this work is to detect the presence of the warp in the RR Lyrae (RRL) population of the Milky Way disc.

Methods. We used a compilation of the three largest public catalogues of RRL stars, precise photometric distances (∼5%), and Gaia DR3 proper motions to kinematically select a sample of thin disc RRLs in the Galactic anti-centre, where the tangential velocity best approximates the azimuthal velocity to differentiate between those that rotate (disc) and those that do not (halo). For disc-like RRLs (321), we analysed their mean vertical height and mean vertical velocity.

Results. For the first time, we show that RRL stars with thin disc-like kinematics trace the warp. In the anti-centre direction, the RRL population reaches a minimum in mean vertical height of ≈0.4 ± 0.2 kpc, with a trend systematically lower than the one found with classical Cepheids. The kinematical signal of the RRL warp starts at R ≈ 10 kpc and, rather than resembling that of the Cepheids, shows a similar trend to the red clump population from previous works, reaching a maximum value of ≈9 ± 5 km s⁻¹ in vertical velocity. We also obtain an estimation of the pattern speed of the RRL warp with a prograde rotation of ≈13 ± 2 km s⁻¹ kpc⁻¹, which is compatible with results obtained from classical Cepheids. Finally, we also obtain a vertical velocity dispersion ≈17 km s⁻¹, which is inconsistent with the kinematics of a canonical old age (>10 Gyr) disc population and, instead, favours a population dominated by intermediate age stars (3–4 Gyr), in agreement with recent works that suggest the existence of such unexpected intermediate age RRLs in the thin disc.

Conclusions. Our results indicate that thin disc RRL stars are a dynamical intermediate-age tracer of the warp, which opens a new window to study the dependency of the warp on stellar age in the Milky Way. The warp’s age dependency will help constrain the physical mechanism behind its origin and its role in the Milky Way dynamical history.