Relationship between the Elemental Abundances and the Kinematics of Galactic-Field RR Lyrae Stars

TL;DR

This study investigates the relationship between chemical abundances and kinematic properties of 415 RR Lyrae stars, revealing insights into their Galactic subsystem memberships and the formation history of the Milky Way.

Contribution

It provides a detailed analysis of elemental abundances and kinematics of RR Lyrae stars, highlighting their diverse Galactic origins and challenging traditional age assumptions.

Findings

RR Lyrae stars show abundance patterns similar to field dwarfs.

Presence of thin disk RR Lyrae stars suggests they are not exclusively old.

Chemical signatures indicate some stars are likely extragalactic in origin.

Abstract

Data of our compiled catalog containing the positions, velocities, and metallicities of 415 RR~Lyrae variable stars and the relative abundances [el/Fe] of 12~elements for 101 RR~Lyrae stars, including four $\alpha$~elements (Mg, Ca, Si, and Ti), are used to study the relationships between the chemical and spatial--kinematic properties of these stars. In general, the dependences of the relative abundances of $\alpha$~elements on metallicity and velocity for the RR~Lyrae stars are approximately the same as those for field dwarfs. Despite the usual claim that these stars are old, among them are representatives of the thin disk, which is the youngest subsystem of the Galaxy. Attention is called to the problem of low-metallicity RR~Lyrae stars. Most RR~Lyrae stars that have the kinematic properties of thick disk stars have metallicities ${\rm [Fe/H]} < -1.0$ and high ratios [$\alpha$/Fe]$ \approx 0.4$, whereas only about 10\,\% of field dwarfs belonging to the so-called "low-metallicity tail" have this chemical composition. At the same time, there is a sharp change in [$\alpha$/Fe] in RR~Lyrae stars belonging just to the thick disk, providing evidence for a long period of formation of this subsystem. The chemical compositions of SDSS J1707+58, V455 Oph, MACHO 176.18833.411, V456 Ser, and BPS CS 30339--046 do not correspond to their kinematics. While the first three of these stars belong to the halo, according to their kinematics, the last two belong to the thick disk. It is proposed that they are all most likely extragalactic, but the possible appearance of some of them in the solar neighborhood as a result of the gravitational action of the bar on field stars cannot be ruled out.