CEPHEID VARIABLES IN NGC 1866: PHOTOMETRIC WORKFLOW, CMD PLOTTING, AND P–L RELATION CONSTRUCTION WITH HST AND TESS OBSERVATIONS

Abstract

This study presents a comprehensive analysis of Classical Cepheid variable stars within the young, metal-rich Globular Cluster NGC 1866. High-resolution imaging from the Hubble Space Telescope (HST) in the F555W and F814W bands was processed using advanced Point Spread Function (PSF) photometry, enabling the resolution of over 16,000 stars per filter in the crowded cluster field and the creation of precise photometric catalogues. The methodology integrates photometric data reduction, colour-magnitude diagram (CMD) construction, Cepheid candidate identification, and the derivation of the Period-Luminosity (P- L) relation. The resulting catalogues were cross-matched to compute accurate absolute magnitudes and colour indices for each star. CMDs were generated using this data, enabling identification of the Instability Strip within NGC1866 and the selection of 113 initial Cepheid candidates based on their CMD positions. To confirm variability and determine pulsation periods, time-series photometric data from the Transiting Exoplanet Survey Satellite (TESS) were extracted for the identified candidates. Lomb-Scargle periodogram analysis of the TESS light curves led to the confirmation of three Classical Cepheids in NGC 1866. Combined with previously published data, this enabled the construction of a robust P-L relation specific to NGC 1866. Linear regression on this sample demonstrated a tight and linear P-L relation with a root mean square (RMS) scatter of 0.072, underscoring its consistency and reliability with respect to Leavitt’s law within this cluster.