Radial velocities of RR Lyrae stars in and around NGC 6441

TL;DR

This study measures radial velocities of RR Lyrae stars around NGC 6441 to identify potential escaped cluster members and understand the cluster's contribution to the Galactic bulge.

Contribution

It provides the first radial velocity measurements of RR Lyrae stars in and around NGC 6441, identifying potential extra-tidal members and analyzing their properties.

Findings

13 RRLs within the tidal radius are confirmed cluster members.

Eight RRLs outside the tidal radius have velocities consistent with cluster membership.

Most potential extra-tidal RRLs align with the cluster's orbit, suggesting they are dissolving cluster stars.

Abstract

Detailed elemental abundance patterns of metal-poor ([Fe/H] ~ -1~dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of globular clusters. Here an attempt is made to identify such presumptive stripped stars originating from the massive, inner Galaxy globular cluster NGC~6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of forty RRLs centered on the globular cluster NGC~6441. All of the 13 RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 +- 5~km/s and a star-to-star scatter of 11~km/s. This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extra-tidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC~6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster's orbit. Therefore, either the tidal radius of NGC~6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC~6441 that are building up the old spheroidal bulge.