Classical Cepheid Candidates in the Globular Cluster NGC 121: A Multi-mission Study with HST and TESS

Classical Cepheids are set of Pulsating Stars (PSs) with high luminosity. They are crucial standard candles for measuring cosmic distances because of their precise Period-Luminosity (P-L) relation. While they are a staple in cosmology, their presence in ancient, metal-poor environments like Globular Clusters (GCs) is rare and provides a unique opportunity to test the universality of the P-L relation. This paper describes a focused search for Classical Cepheid candidates within the ancient globular cluster NGC 121. The methodology employed a multi-mission approach, combining high-resolution photometry from the Hubble Space Telescope (HST) with time-series data from the Transiting Exoplanet Survey Satellite (TESS). Point Spread Function (PSF) photometry was used to identify individual stars from HST photometry FITS (Flexible Image Transport System) images and to create comprehensive stellar catalogues. The identified stars were plotted in a Colour - Magnitude Diagram (CMD) and the instability strip was identified on the CMD. Then a variability analysis was performed using the Lomb-Scargle periodogram on their TESS light curves to confirm their pulsation. This process successfully identified five stars recognized as candidates for Classical Cepheids, a significant discovery given the cluster’s advanced age and low metallicity. The existence of these candidates provides crucial data for investigating the reliability of the P-L relation in non-traditional stellar environments for Classical Cepheids. These findings highlight the critical importance of combining high-quality spatial and temporal data to ensure accurate identification and classification in challenging astronomical settings. This work not only contributes a new catalogue of rare candidates but also reinforces the ongoing effort to refine the cosmic distance scale.