Zwicky Transient Facility and Globular Clusters: The RR Lyrae gri-Band Period-Luminosity-Metallicity and Period-Wesenheit-Metallicity Relations

TL;DR

This paper derives new period-luminosity-metallicity and related relations for RR Lyrae stars in globular clusters using ZTF data, improving distance measurements and understanding of these variable stars.

Contribution

The study presents the first derivation of period-Wesenheit-metallicity and period-color-metallicity relations for RR Lyrae in globular clusters from ZTF data.

Findings

Derived PLZ and PWZ relations in gri bands for ~750 RR Lyrae.

Compared relations with literature, finding good agreement.

Applied relations to Crater II, suggesting a larger distance modulus.

Abstract

Based on time-series observations collected from Zwicky Transient Facility (ZTF), we derived period-luminosity-metallicity (PLZ) and period-Wesenheit-metallicity (PWZ) relations for RR Lyrae located in globular clusters. We have applied various selection criteria to exclude RR Lyrae with problematic or spurious light curves. These selection criteria utilized information on the number of data points per light curve, amplitudes, colors, and residuals on the period-luminosity and/or period-Wesenheit relations. Due to blending, a number of RR Lyrae in globular clusters were found to be anomalously bright and have small amplitudes of their ZTF light curves. We used our final sample of ~750 RR Lyrae in 46 globular clusters covering a wide metallicity range (-2.36 < [Fe/H] < -0.54 dex) to derive PLZ and PWZ relations in gri bands. In addition, we have also derived the period-color-metallicity (PCZ) and for the first time, the PQZ relations where the Q-index is extinction-free by construction. We have compared our various relations to empirical and theoretical relations available in literature, and found a good agreement with most studies. Finally, we applied our derived PLZ relation to a dwarf galaxy, Crater II, and found its true distance modulus should be larger than the most recent determination.