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

Abstract

We present the first gri-band period–luminosity (PL) and period–Wesenheit (PW) relations for 37 Type II Cepheids (TIICs) located in 18 globular clusters based on photometric data from the Zwicky Transient Facility. We also updated BVIJHK-band absolute magnitudes for 58 TIICs in 24 globular clusters using the latest homogeneous distances to the globular clusters. The slopes of g/r/i- and B/V/I-band PL relations are found to be statistically consistent when using the same sample of distance and reddening. We employed the calibration of ri-band PL/PW relations in globular clusters to estimate a distance to M31 based on a sample of ∼270 TIICs from the PAndromeda project. The distance modulus to M31, obtained using calibrated ri-band PW relation, agrees well with the recent determination based on classical Cepheids. However, distance moduli derived using the calibrated r- and i-band PL relations are systematically smaller by ∼0.2 mag, suggesting there are possible additional systematic errors on the PL relations. Finally, we also derive the period–color (PC) relations and for the first time the period–Q-index (PQ) relations, where the Q-index is reddening free, for our sample of TIICs. The PC relations based on (r − i) and near-infrared colors and the PQ relations are found to be relatively independent of the pulsation periods.

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

“…On the other hand, the Sloan Digital Sky Survey (SDSS) and the SDSS-variant ugriz filters (or a subset of them) are now becoming more popular in a number of timeseries synoptic sky surveys, such as the representative Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST; Ivezić et al 2019). Similar to our previous work on contact binaries (Ngeow et al 2021), RR Lyrae (Ngeow et al 2022a), and Type II Cepheids (Ngeow et al 2022b), we aimed to derive the gri-band PL and PW relations for ACep using homogeneous gri-band data obtained from the Zwicky Transient Facility (ZTF; Bellm & Kulkarni 2017;Bellm et al 2019;Graham et al 2019;Dekany et al 2020). ZTF repeatedly observes the northern sky in customized gri filters with a dedicated 47 squared-degree wide-field mosaic CCD camera mounted on the Palomar 48 inch Samuel Oschin Schmidt telescope.…”

“…Due to large number of available data points per light curves (see Figure 1), we estimated negligible errors on these mean magnitudes. We note that this two-step process is identical to the one adopted for the Type II Cepheids studied in Ngeow et al (2022b). The left panel of Figure 1 presents the folded ZTF light curves for this ACep.…”

“…We first used the LombScargleMultiband module from the astroML/gatspy 8 package (VanderPlas & Ivezić 2015) to search the period of this ACep on the ZTF griband light curves. A low-order Fourier expansion was used to fit the folded light curves (for more details, see Ngeow et al 2022b, and reference therein), and outliers beyond 3 s from the fitted light curves were identified and removed (where s is the dispersion of the fitted light curve). We then ran the second pass of the LombScargleMultiband module to determined the final adopted period, P = 0.821322 days, for NGC5466 V19.…”

“…The numbers of data points in the gri-band light curves are 410, 630, and 180 for NGC5466 V19, and 1433, 1402, and 432 for M92 V7. The black crosses are rejected outliers based on a two-step process as described in the text (see alsoNgeow et al 2022b). The black curves are the fitted low-order Fourier expansion to the remaining data points on the light curves.…”

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for SX Phoenicis Variables in the gri Band

“…Similar to our previous work (Ngeow et al 2022b), we refined the pulsation periods for the 121 SXP stars, selected in previous section with sufficient number of observations, using the LombScargleMultiband (LSM) module (VanderPlas & Ivezić 2015) 9 in a two-step process. The first step is running the LSM module on the gri-band (whenever available) ZTF light curves for each SXP stars within a period range of 0.005-0.500 days.…”