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

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.

Figures (3)

• Figure 1: Comparison of distances. Panel $a)$$d_I$ vs $d_{UV}$, panel $b)$$d_P$ vs $d_{UV}$ and panel $c)$$d_I$ vs $d_P$. See § \ref{['sec:Results']} for a discussion
• Figure 2: Comparison of metallicities. Panel $a)$ [Fe/H]$_J$ vs [Fe/H]$_{\rm UVES}$, panel $b)$ [Fe/H]$_D$ vs [Fe/H]$_{\rm UVES}$ and panel $c)$ [Fe/H]$_J$ vs [Fe/H]$_D$. The blue lines are least-squares fits to the distribution of black points. The systematic shift of [Fe/H]$_J$ and [Fe/H]$_D$ relative to the spectroscopic values [Fe/H]$_{\rm UVES}$ are evident. The red squares in panel $a)$ correspond to the metallicities [Fe/H]$^{\rm UV}_{\rm JK}$ vs [Fe/H]$_{\rm UVES}$ calculated via the Fourier decompositions of OGLE $V$ light curves of RR Lyrae stars satisfying the inclusion criteria in clusters NGC 6522, NGC 6544 and Djorg 2 (see Table \ref{['Results2']}). They show a better agreement with the spectroscopic results. See § \ref{['sec:Results']} for a detailed discussion.
• Figure 3: The spatial distribution of the analyzed globular clusters in Cartesian coordinates, using mean distances from Prudil2025 in the top panel, and orbital energy vs. angular momentum in the bottom panel. The black lines in the top panel show surface-density contours of an analytical potential implemented in the AGAMA software Vasiliev2019Agama, for a stellar component rotated by a bar angle of 25$^\circ$. The colored points in the bottom panel depict analysed globular clusters Prudil2025Kin in this study, with color-coding based on association with one of the MW substructures or accreted structures. For a handful of displayed globular clusters, we provide their associated labels.