Ultra Long Period Cepheids as standard candles from Gaia to Rubin-LSST

TL;DR

This study analyzes Ultra Long Period Cepheids using Gaia and Rubin-LSST data, suggesting they could serve as standard candles to improve cosmic distance measurements and address the Hubble constant tension.

Contribution

It provides the largest ULP sample with new Gaia DR3 photometry and demonstrates the potential of Rubin-LSST to enhance ULP observations and their use as standard candles.

Findings

ULP Period-Wesenheit relation has reduced dispersion with improved photometry.

ULPs are likely an extension of Classical Cepheids at higher periods and luminosities.

Rubin-LSST can significantly improve ULP data quality for cosmological applications.

Abstract

An analysis of the Ultra Long Period Cepheids (ULPs) properties 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 Long Period Variable in Gaia DR3, but recently reclassified as 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-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.