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

Abstract

Based on time-series observations collected from the 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 dex < [Fe/H] < −0.54 dex) to derive PLZ and PWZ relations in the gri bands. In addition, we have also derived the period–color–metallicity and, for the first time, the period-Q-index-metallicity relations, where the Q-index is extinction-free by construction. We have compared our various relations to empirical and theoretical relations available in the literature and found a good agreement with most studies. Finally, we applied our derived PLZ relation to a dwarf galaxy, Crater II, and found that its true distance modulus should be larger than the most recent determination.

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for Type II Cepheids

“…In contrast, the (B − I) PC relation displays the largest dispersion among all the PC relations. In case of the PQ relations, slopes for both of the Q JHK and Q gri PQ relations are statistically consistent with zero, in contrast to the RR Lyrae (Ngeow et al 2022). The Q BVI PQ relation is also much shallower than the BVI-band PC relations, and has the smallest dispersion among the three PQ relations.…”

“…Ngeow et al (2022) as Q gri = (g − r) − 1.395(r − i). The fitted PW and PC/PQ relations are summarized in Tables…”

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for Anomalous Cepheids Supplemented with Large Magellanic Cloud Sample

“…The mean magnitudes provided in Vivas et al (2020) are in the SDSS photometric system. Therefore, following the procedures described in Ngeow et al (2022a), we transformed our gi-band PL relations to the SDSS system by adding a small correction term to our PL relations. These correction terms were determined using a subset of calibrating ACep with the same mean (g − i) SDSS colors as the fundamental mode ACep in Crater II.…”

Evaluating the V-band Photometric Metallicity with Fundamental Mode RR Lyrae in the Kepler Field

“…To remedy this, we provided a template color curve in Appendix B, such that colors can be estimated at various pulsational phases using the template color curve and an estimation of the mean color, and not assuming a constant or zero color. There are various approaches to estimate the mean color for an ab-type RR Lyrae, such as using the observed light curves, using prior information (e.g., from other surveys or observations), using a period-color relation (e.g., in Ngeow et al 2022), etc. It is up to the researchers to decide which approach to use, depending on their situations, needs, and goals.…”