Zwicky Transient Facility and Globular Clusters: the Period-Luminosity and Period-Luminosity-Color Relations for Late-Type Contact Binaries

TL;DR

This study derives and calibrates period-luminosity and period-luminosity-color relations for late-type contact binaries in globular clusters using ZTF data, enhancing understanding of these variables across different environments.

Contribution

First derivation of $gri$-band PL and PLC relations for late-type contact binaries in globular clusters, combining data from globular clusters and the Solar neighborhood.

Findings

PL and PW relations are consistent across samples.

Derived relations show improved accuracy over previous limited data.

Relations can be used for distance measurements and stellar population studies.

Abstract

In this work, we aimed to derive the $gri$-band period-luminosity (PL) and period-luminosity-color (PLC) relations for late-type contact binaries, for the first time, located in the globular clusters, using the homogeneous light curves collected by the Zwicky Transient Factory (ZTF). We started with 79 contact binaries in 15 globular clusters, and retained 30 contact binaries in 10 globular clusters that have adequate number of data points in the ZTF light curves and unaffected by blending. Magnitudes at mean and maximum light of these contact binaries were determined using a fourth-order Fourier expansion, while extinction corrections were done using the {\tt Bayerstar2019} 3D reddening map together with adopting the homogeneous distances to their host globular clusters. After removing early-type and "anomaly" contact binaries, our derived $gri$-band PL and period-Wesenheit (PW) relations exhibit a much larger dispersion with large errors on the fitted coefficients. Nevertheless, the $gr$-band PL and PW relations based on this small sample of contact binaries in globular clusters were consistent with those based on a larger sample of nearby contact binaries. Good agreements of the PL and PW relations suggested both samples of contact binaries in the local Solar neighborhood and in the distant globular clusters can be combined and used to derive and calibrate the PL, PW and PLC relations. The final derived $gr$-band PL, PW and PLC relations were much improved than those based on the limited sample of contact binaries in the globular clusters.