Near-infrared observations of RR Lyrae and Type II Cepheid variables in the metal-rich bulge globular cluster NGC 6441

Abstract

NGC 6441 is a bulge globular cluster with an unusual horizontal branch morphology and a rich population of RR Lyrae (RRL) and Type II Cepheid (T2C) variables that is unexpected for its relatively high metallicity. We present near-infrared (NIR, $JH{K}_s$) time-series observations of 42 RRL, 8 T2Cs, and 10 eclipsing binary candidate variables in NGC 6441. The multi-epoch observations were obtained using the FLAMINGOS-2 instrument on the 8-m Gemini South telescope. Multi-band data are used to investigate pulsation properties of RRL and T2C variables including their light curves, instability strip, period--amplitude diagrams, and period--luminosity and period--wesenheit relations (PLRs and PWRs) in $JH{K}_s$ filters. NIR pulsation properties of RRL variables are well fitted with theoretical models that have canonical helium content and mean-metallicity of NGC 6441 ([Fe/H]$=-0.44\pm 0.07$~dex). The helium-enhanced RRL models predict brighter NIR magnitudes and bluer colors than the observations of RRL in the cluster. This suggests that these RRL variables in NGC 6441 are either not significantly helium enhanced as previously thought or the impact of such enhancement is smaller in NIR than the predictions of the pulsation models. We also use theoretical calibrations of RRL period--luminosity--metallicity (PLZ) relations to simultaneously estimate the mean reddening, $E(J-K_s)=0.26\pm 0.06$~mag, and the distance, $d=12.67\pm 0.09$~kpc, to NGC 6441. Our mean reddening value and the distance are consistent with the independent estimates from the bulge reddening map based on red clump stars and the latest Gaia data, respectively. Our distance and reddening values provide a very good agreement between the PLRs of T2Cs in NGC 6441 and those for RRL and T2Cs in Galactic globular clusters that span a broad range of metallicity.

17 pages, 14 figures, accepted in A&A, abridged abstract

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• A theoretical framework for BL Her stars III. A case study: Robust light curve optimisation in the LMC
• Period-Luminosity Relations for Galactic Type II Cepheids in the Sloan bands
• A theoretical framework for BL Her stars IV. New period-luminosity relations in the Rubin-LSST filters