Application of interferometry to the study of Cepheids and dwarf stars

Optical interferometry is a powerful technique for the study of stars in exquisite details, by enabling the spatial resolution of their disks. I present three applications of this technique to the study of different types of stars: Cepheids, main sequence stars, and fast rotators. Cepheids are a famous class of supergiants that are widely used as standard candles for distance measurements, through the period-luminosity (P-L) relations. Interferometry allows to apply a refined version of the Baade-Wesselink method to estimate the distance to nearby Cepheids, and therefore calibrate the P-L relations. While this method is promising and potentially very precise, several points in its application are critical, in particular the projection factor. Main sequence stars are by far the most numerous stellar class in the universe, as all stars spend the majority of their life as dwarfs. I have applied interferometry to a broad range of main sequence stars, from the very low mass Proxima to hot stars. In particular, I have realized the first combination of interferometric and asteroseismic constraints for the modeling of stars (the two components of alpha Centauri, as well as other stars). In a third part, I describe the recent results that we have obtained on fast rotating stars. Due to the very strong centrifugal force at the equator, the photosphere of these stars is strongly distorted. Interferometry allows to observe these deformations, and I describe our observations of three bright, fast rotating stars: Altair (alpha Aql), Achernar (alpha Eri) and Vega (alpha Lyr).