Local RR Lyrae Stars: native and alien

TL;DR

This study analyzes the metallicity and kinematics of 464 RR Lyrae stars within 6 kpc of the Sun, revealing a thin disc population and evidence of a massive merger remnant similar to Gaia-Sausage/Gaia-Enceladus.

Contribution

It provides a homogeneous metallicity scale for RR Lyrae stars and links their kinematics to a significant past merger event in the Milky Way.

Findings

Confirmation of a thin-disc RR Lyrae population with solar alpha/Fe ratios.

Identification of halo RR Lyrae stars with energies and angular momenta consistent with Gaia-Sausage/Gaia-Enceladus debris.

Evidence suggesting the disrupted galaxy was as massive as or larger than the Large Magellanic Cloud.

Abstract

Measurements of [Fe/H] and radial velocity are presented for 89 RR Lyrae (RRL) candidates within 6 kpc of the Sun. After removal of two suspected non-RRL, these stars were added to an existing database, which yielded 464 RRL with [Fe/H] on a homogeneous scale. Using data from the Gaia satellite (Data Release 2), we calculated the positions and space velocities for this sample. These data confirm the existence of a thin-disc of RRL with [alpha/Fe] ~ solar. The majority of the halo RRL with large total energies have near zero angular momenta about the Z axis. Kinematically these stars closely resemble the Gaia-Sausage/Gaia-Enceladus stars that others have proposed are debris from the merger of a large galaxy with the Milky Way. The metallicity and period distributions of the RRL and their positions in the period-amplitude diagram suggest that this disrupted galaxy was as massive as the Large Magellanic Cloud and possibly greater.