Dust at the extreme: the CHARA Array polarization model and prospects for polarimetric interferometry

Abstract

Polarimetric interferometry combines milliarcsecond angular resolution with sensitivity to scattered light and can therefore probe both dust formation around evolved stars and dust survival at the inner rims of planet-forming disks. At optical and near-infrared wavelengths, however, the many oblique reflections in an interferometric beam train introduce substantial, time-dependent instrumental polarization. We summarize the CHARA Array polarization model,1 which uses Jones matrices to describe the diattenuation and retardance of the telescopes, Coud´e trains, delay lines, and laboratory optics while accounting for field rotation during target tracking. The model is constrained jointly by per-telescope flux ratios, per-baseline visibility ratios, and differential phases and calibrates the transfer function to 3.4% in visibility ratio and 1.4◦ in differential phase for MIRC-X in the H band, and to 5.9% and 2.4◦, respectively, for MYSTIC in the K band. The fitted parameters also identify telescope-dependent retardance and chromatic birefringence in the laboratory optics. We describe the measurements needed to recover linear Stokes visibilities and outline observing strategies for spatially resolved studies of circumstellar dust. In the future, such Stokes measurements can be extended to the K band with VLTI/GRAVITY and used to reconstruct polarized interferometric images.