Interferometric measurements of stellar intensity profiles

Abstract

We report on direct interferometric measurements of stellar intensity profiles obtained with the Navy Prototype Optical Interferometer (NPOI) and the Very Large Telescope Interferometer (VLTI). These measurements need to make use of weak fringes, i.e. low visibility values, on resolved stars. We describe techniques that were used to obtain these low visibility values with high precision. They include the methods of baseline bootstrapping and wavelength bootstrapping, as well as, lately, coherent integration using phase bootstrapping. In addition, we developed methods to compensate photon and detection biases. We present recent measurements on the giant star γ Sge, obtained with the NPOI, which succeeded not only in discriminating between uniform disks and limb-darkened disks, but also in constraining Kurucz model atmosphere parameters. We present first VLTI measurements of visibility values beyond the first minimum which were taken on the giant star Ψ Phe. Here, the capabilities to synthesize baselines of different lengths and to use different aperture sizes were used for the first time with the VLTI. We close with an outlook on the future potential on studies of stellar surface structure with the six-way beam combination at the NPOI and with the completed VLTI. This includes for instance direct measurements of the limb-darkened profiles of a large number of different types of stars, and of starspots which may for instance be caused by magnetic fields or large-scale photospheric convection.