Title of the thesis
Optimised modelling of pulsating stars for the extragalactic distance scale.
Composition of the jury
- Yveline Lebreton (LIRA) : Chair ;
- Katrien Kolenberg (KU Leuven) : rapporteur ;
- Denis Mourard (OCA) : rapporteur
- Louise Breuval (STScI) : examiner ;
- Grzegroz Pietrzyński (CAMK) : examiner ;
- Antoine Mérand (ESO) : invited member ;
- Nicolas Nardetto (OCA) : invited member ;
- Pierre Kervella (LIRA) : supervisor
Abstract
Pulsating variable stars, particularly Cepheids and RR Lyrae stars, are powerful tools for determining distances. Period-luminosity relations allow us to deduce their intrinsic luminosity, which is then compared to their apparent magnitude. In addition, the parallax-of-pulsation method uses the trigonometric relationship between the radius variation and the angular diameter variation to determine the distance. This latter technique involves a parameter, the projection factor, which converts the observed radial velocity to the photospheric pulsation velocity. This parameter, which is entirely degenerate with distance, shows significant statistical variations, preventing us from using parallax-of-pulsation on a large scale. A more detailed understanding of pulsation is required in order to determine the origin of this dispersion.
In this thesis, I first study the projection factor of RR Lyrae stars, using the SPIPS code and new radial velocity measurements, in order to compare it with Cepheids. I show that the value and dispersion of the projection factor for RR Lyrae stars are quite similar to what has been observed for Cepheids and that the different methods of measuring radial velocities have little impact on this dispersion, provided that homogeneous data are considered. I also present my work on the variation of the centre-to-limb darkening coefficient during pulsation, a key ingredient of the projection factor, using interferometric observations of the Cepheid η Aql.
Secondly, I develop generic light curves for Cepheids using SPIPS multi-band models. These templates make it possible to predict the shape of the light curve for a given pulsation period in a large variety of photometric bands. They are particularly effective for modelling light curves based on rare or imprecise observations, as is the case with extragalactic Cepheids.
