Distance and [Fe/H] of Galactic bulge clusters from member RR Lyrae I-band light curves

TL;DR

This study derives [Fe/H] and distances for 24 Galactic bulge globular clusters using RR Lyrae I-band light curves, comparing results with V-band calibrations and analyzing their orbital properties within the Milky Way.

Contribution

It introduces I-band Fourier calibrations for RR Lyrae stars to determine cluster metallicities and distances, and assesses their orbital characteristics in the Galactic bulge.

Findings

I-band calibrations yield [Fe/H] values within -0.29 to +0.15 dex of UVES spectroscopic scale.

PMZ distances agree within 0.4 kpc with recent distance compilations.

Bulge clusters show low angular momentum and energies, indicating early formation in the Milky Way.

Abstract

We have investigated the results for [Fe/H] and distance for a group of 24 globular clusters in the Galactic bulge, employing recent calibrations of RR Lyrae light curves Fourier decomposition and period-absolute magnitude-metallicity (PMZ) calibrations in the I-band. We have limited our calculations to RR Lyrae stars that have been proven to be very likely cluster members. These results are compared with [Fe/H] and Mv (distance) obtained from well-established Fourier calibrations in the V-band. These calibrations of the I-band were found to produce iron values that can differ from the UVES spectroscopic scale by -0.29 to +0.15 dex. The PMZ distances agree within 0.4 kpc with recent solid critical distance compilations. Adopting the newly derived distances, we conducted a spatial and orbital analysis of the bulge globular clusters in a non-axisymmetric Milky Way potential, and compared their orbital properties with earlier studies, finding broadly consistent trends with small systematic differences driven by the assumed distances and Galactic model. Clusters associated with the in situ bulge component display a narrow range low angular momentum and low orbital energies, consistent with formation in the early inner Milky Way.