No Evidence of Polarization in the 11.3 <i>μ</i> m Polycyclic Aromatic Hydrocarbon Emission Line by Independent Analyses

Abstract Polycyclic aromatic hydrocarbons (PAHs) are commonly used as proxies for star formation, molecular gas content, and other interstellar medium (ISM) properties in our Galaxy and other galaxies. Given their abundance and brightness, polarization measurements of PAH features could, in principle, provide a probe of the ISM magnetic field and intrinsic PAH properties; however, the diagnostic power of PAH polarization remains to be established. Previous studies reported that the 11.3 μ m PAH emission line in the northwestern nebula of the Herbig Be star MWC 1080 was polarized at 1.9% ± 0.2%. This level of polarization was explained via the paramagnetic relaxation process, which may allow the characterization of magnetic fields in the ISM. Using the same observations, here, we reanalyzed the 8–13 μ m spectropolarimetric observations taken with CanariCam on the 10.4 m Gran Telescopio CANARIAS (GTC), and we measure a polarization of 0.5% ± 0.6% within 11.3 ± 0.2 μ m, consistent with an unpolarized source, 0.6% ± 0.2% (instrumental polarization). We reproduce the previously reported polarized PAH emission line if the polarization fraction spectrum is oversubtracted by a constant instrumental polarization and the polarization uncertainties, which is inconsistent with the fundamentals of polarimetric data analysis. Thus, the published 8–13 μ m spectropolarimetric data taken with CanariCam/GTC provide no statistical evidence for a polarized 11.3 μ m PAH emission line, in agreement with current dust models.