Infrared Imaging of Capella with the IOTA Closure Phase Interferometer

We present infrared aperture synthesis maps produced with the upgraded Infrared Optical Telescope Array interferometer. Michelson interferograms on the close binary system Capella (α Aur) were obtained in the H band between 2002 November 12 and 16 using the IONIC3 beam combiner. With baselines of 15 m ≤ B ≤ 38 m, we were able to determine the relative position of the binary components with milliarcsecond precision and to track their movement along the ≈14° arc covered by our observation run. We briefly describe the algorithms used for visibility and closure phase estimation. Three different hybrid mapping and bispectrum fitting techniques were implemented within one software framework and used to reconstruct the source brightness distribution. By dividing our data into subsets, the system could be mapped at three epochs, revealing the motion of the stars. The precise position of the binary components was also determined with model fits, which in addition revealed I_Aa/I_Ab = 1.49 ± 0.10 and apparent stellar uniform-disk diameters of Θ_Aa = 8.9 ± 0.6 mas and Θ_Ab = 5.8 ± 0.8 mas. To improve the (u,v)-plane coverage, we compensated this orbital motion by applying a rotation-compensating coordinate transformation. The resulting model-independent map with a beam size of 5.4 mas × 2.6 mas allows the resolution of the stellar surfaces of the Capella giants themselves.