Variable stars in the bulge globular cluster NGC 6401

TL;DR

This study identifies and characterizes variable stars in the globular cluster NGC 6401, providing new data on its stellar population, metallicity, and distance despite challenges from differential reddening.

Contribution

The paper offers the first detailed catalog of RR Lyrae stars in NGC 6401, including their properties and implications for the cluster's metallicity and Oosterhoff classification.

Findings

23 RRab and 11 RRc stars identified

Mean metallicity [Fe/H]$_{ ext{UVES}}$ = -1.13

Distance approximately 6.35 kpc

Abstract

We present a study of variable stars in globular cluster NGC 6401. The cluster is only $5.3\degr$ away from the Galactic centre and suffers from strong differential reddening. The photometric precision afforded us by difference image analysis resulted in improved sensitivity to variability in formerly inaccessible interior regions of the cluster. We find 23 RRab and 11 RRc stars within one cluster radius (2.4$\arcmin$), for which we provide coordinates, finder-charts and time-series photometry. Through Fourier decomposition of the RR Lyrae star light curves we derive a mean metallicity of [Fe/H]$_{\mathrm{UVES}} = -1.13 \pm 0.06$ (${\rm [Fe/H]}_{\mathrm{ZW}} = -1.25 \pm 0.06$), and a distance of $d\approx 6.35 \pm 0.81$ kpc. Using the RR Lyrae population, we also determine that NGC 6401 is an Oosterhoff type I cluster.