Abstract
Optical interferometry is the only technique giving access to milli-arcsecond resolution at optical/infrared wavelengths. For Cepheids, this is a powerful and unique tool to measure distances in a pseudo-geometric way, and probe the close circumstellar environment. While the measured mean angular diameter allows us to probe the pulsation mode, its angular and linear variation can provide the distance to the star. Independent distance measurements are particularly important because classical Cepheids are used as primary distance indicator in the Local Group. Interferometry also offers the possibility to probe the close environment of these stars, and so study their circumstellar envelopes (CSEs) and the companions. The characterization of the CSEs is particularly important as they give access to the present mass loss rate of Cepheids. These CSEs were probably formed through past or ongoing mass loss, possibly generated by shock waves in the pulsating atmosphere of the Cepheid. Their presence can also bias the distance estimate through the Baade-Wesselink method. Finally, when Cepheids are in binary systems, we can investigate their age and evolution, estimate the mass and distance, and constrain evolution and pulsation models. However, most of the companions are located too close to the Cepheid (∼ 1-40 mas) to be observed with a 10-meter class telescope. The only way to spatially resolve such systems is to use long-baseline interferometry or aperture masking.
