De Somma, Giulia (2021) Classical Cepheids: an updated theoretical scenario in the Gaia Era. [Tesi di dottorato]

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Classical Cepheids are the most important primary distance indicators, and excellent tracers of relatively young and intermediate-age (from a few tens to a few hundreds of Myr) stellar populations. Indeed, they are well known to obey period-luminosity and period-luminosity-color relations which are traditionally used to calibrate secondary distance indicators and, hence, to estimate the Hubble constant. Stellar evolution predicts that the evolutionary phase of Classical Cepheids corresponds to the central helium-burning phase of massive and intermediate-mass stars. Stars in this evolutionary phase obey a mass-luminosity relation which is dependent on the chemical composition as well efficiency of some non canonical physical processes such as rotation, core convective overshooting, and mass loss efficiency during (mainly) the red giant branch stage. The mass-luminosity relation, adopted in the pulsational model computation, affects the shape of light curves and radial velocity curves, the coefficients of PLC and period-Wesenheit relations and, in turn, the Cepheid-based distance scale. The main focus of this PhD project is the modelling of radially pulsating stars, specifically, Classical Cepheids, through non linear hydrodynamical models. The primary goal is to constrain the extragalactic distance scale, and test the impact of the various ingredients entering the model computation, on the theoretical calibration of the Classical Cepheid distance scale. This PhD research was conducted, in the context of one of the most debated issues in current astrophysical literature: the Hubble Constant tension which is the discrepancy (at the level of 4.4 $\sigma$) between the value of the Hubble constant derived by Riess et al (2016, 2018) on the basis of Classical Cepheids and the Cosmic Microwave Background results. Indeed, the first part of this research is devoted to the evaluation of possible residual systematic errors on the cosmic Cepheid-based distance scale calibration and Hubble constant evaluation, in the context of one of the most debated issues in current astrophysical literature: the Hubble constant tension. The computation of updated and reliable pulsating stellar models for Classical Cepheids is the basis for the derivation of theoretical tools to constrain not only the distance but also intrinsic stellar properties such as the age and the mass of observed pulsators. Moreover, their comparison with valuable observational datasets, including that provided by the Gaia mission, allows us to test the predictive capability of current pulsation theory, and to trace the properties of Galactic and extra-galactic young and intermediate-age stellar populations. The pulsational framework developed in this PhD thesis is appropriate not only for typical Galactic Classical Cepheids, but also to chemical patterns characteristic of extra-galactic stellar systems such as the Magellanic Clouds. The pulsational scenario presented in this work is also important in order to constrain the physical and numerical assumptions adopted in current generation of stellar evolution models for massive and intermediate-mass stars. In this context, the final part of the PhD thesis is devoted to the use of Classical Cepheids as age indicators. By combining updated stellar evolution models with our updated pulsational scenario, new accurate Classical Cepheid period-age and the period-age-color relations in the Gaia filters are provided. The inferred period–age and period–age–color relations are applied to a selected sample of both fundamental and first overtone mode Cepheids observed with the Gaia spacecraft, and individual ages for the various adopted theoretical scenarios are derived. The thesis will end with a discussion on the first results of the chemical composition effect on the updated and homogeneous theoretical scenario for Classical Cepheids. Accurate metal-dependent relations which represent powerful tools for constraining stellar parameters of Classical Cepheid distances with simultaneous photometric and spectroscopic information in the Local Group of galaxies, are derived. The development of useful tools for constraining the individual distances and intrinsic stellar parameters in a variety of Galactic and extragalactic environments, involves a direct comparison between observed and predicted light curves. Therefore, in the era of Gaia astrometric measurements and in view of the next generation of the most advanced observational facilities, the work presented in the PhD thesis paves the way for future applications and comparisons between theory and observations.

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