The project contributed to the computation of a fine grid of pulsation models for Cepheid and RR Lyrae variables for different chemical compositions representative of these variables in the Milky Way, nearby galaxies such as the Large and the Small Magellanic Cloud. These pulsation models were used to derive theoretical PL and PLZ relations for Cepheid and RR Lyrae stars in different bands relevant to space missions such as Gaia, and upcoming ground-based facilities such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (see e.g. De Somma et al. 2024), that were also tested against observations (see e.g. Bhardwaj et al. 2023) . More specifically, these pulsation models were used to gain new insights into the so-called “Hertzsprung progression” in the light and radial velocity curves of Cepheid variables (Marconi et al. 2024). These models were utilised by the students working with the researcher and host to obtain physical parameters of Cepheid and RR Lyrae stars (Kumar et al. 2023), and are currently being used in several projects within the group. All these models are subsequently provided to the community with the publications.
One of the main objectives of the PRESTiGE project was to evaluate the tension in the Hubble constant measurement based on classical Cepheid Supernovae distance ladder and the value that comes from the Planck mission’s observations of the early Universe. One of the systematics in the Cepheid-based Hubble constant determination has been the metallicity effect on their PL relations. The PRESTiGE project obtained high-resolution spectroscopic metallicities of Milky Way Cepheid standards with available Hubble Space Telescope photometry. In a joint publication with the SH0ES project team (Supernovae and H0 for the Equate of State of the Dark Energy led by the Nobel Laureate Adam Riess), the experienced researcher found that there is no significant impact of Cepheid metallicity or the metallicity scale on the local measurement of the Hubble constant. This publication (Bhardwaj, Riess et. al. 2023) further confirmed that Cepheid metallicities are not at the root cause for the Hubble tension.
The quantification of the impact of metallicity on Cepheid and RR Lyrae pulsation properties has been a crucial issue for stellar astrophysics and distance scale. In line with the objective of the PRESTiGE project, the researcher and host provided new empirical and theoretical calibrations of RR Lyrae PLZ relations at near-infrared wavelengths. The publication (Bhardwaj, Marconi+2023) provided the most precise PLZ relation using nearly 1000 RR Lyrae stars in 11 Milky Way globular clusters and found an excellent agreement with the theoretical predictions and empirical results. Similarly, the researcher and host also derived empirical PLZ relations for classical Cepheids using parallaxes from the Gaia space mission and photometric data collected through their observing programs (Bhardwaj, Ripepi et al. 2024).
