Cepheids Observations Using CHARA/FLUOR: αUMi and δCep

Summary

Over the past decade, stellar interferometry emerged as a powerful tool to measure distances to Cepheids using the pulsation parallax.method, also called the Baade-Wesselink (BW) method. However, as interferometers gain in baseline length and precision, the assumptions on which the interferometric BW method relies have to be refined: the projection factor (to convert radial velocity in pulsation velocity) and the center to limb darkening (to properly interpret interferometric data) have to be studied theoretically and/or observationally. We report here interferometric observations of two classical Cepheids, using the FLUOR beam combiner \cite{Foresto2003} installed at the CHARA Array \cite{tenBrummelaar2005}. αUMi was observed in both the first and the second lobes of the visibility curve, enabling us to potentially measure its center to limb darkening. We measured with high precision the angular diameter variation for δCep. Combined with the distance to this star from HST/FGS cite{Benedict2002}, these measurements provide directly the projection factor