Mid-infrared Polarization of the Diffuse Interstellar Medium toward CygOB2-12

Abstract We present the first mid-IR detection of the linear polarization toward the star CygOB2-12, a luminous blue hypergiant that, with A V ≈ 10 mag of foreground extinction, is a benchmark in the study of the properties of dust in the diffuse interstellar medium. The 8–13 μm spectropolarimetry, obtained with the CanariCam multimode camera at the Gran Telescopio CANARIAS shows clear trends with wavelength characteristic of silicate grains aligned in the interstellar magnetic field. The maximum polarization, detected with 7.8σ statistical significance near 10.2 μm, is (1.24 ± 0.28)% with position angle 126° ± 8°. We comment on these measurements in the context of recent models for the dust composition in the diffuse interstellar medium.