2024ApJS..275...26M

An analysis of the properties of ultra-long-period Cepheids (ULPs) could significantly contribute to understanding the Hubble constant tension, e.g., the current discrepancy between determinations based on local distance indicators and those relying on cosmic microwave background measurements. These highly luminous variables are observable beyond 100 Mpc, so if they were confirmed to behave as standard candles, they would allow us a direct measurement of cosmological distances without any secondary distance indicator, thus reducing potential systematic errors in the calibration of the cosmic distance scale. This paper presents an analysis of the largest known sample of 73 ULPs, including 15 objects in nearby galaxies, with new accurate and homogeneous photometry obtained by Gaia DR3, and a new object, in our Galaxy, identified as a long-period variable in Gaia DR3, but recently reclassified as a ULP. The obtained results suggest that, by improving photometric accuracy, the ULP period-Wesenheit relation shows a smaller dispersion than that obtained in literature and is in better agreement with the classical Cepheid (CC) one, supporting the hypothesis that ULPs are the extension of the CCs at higher period, mass, and luminosity. However, to reach this aim, it is necessary to enrich the sample with high-quality data. The Rubin Observatory Legacy Survey of Space and Time (Rubin-LSST) survey offers the possibility to achieve this thanks to its photometric characteristics and time extension. In particular, we will explore the capabilities of the Rubin-LSST survey to recover ULP theoretical light curves by using a new tool called PulsationStarRecovery, built by our group for this type of analysis.