Inferring Magnetic Field Morphology and Dust Scattering Geometry from Mid-IR Polarimetry: the Extended Aitken Method

Abstract

The Aitken method is a useful approach for decomposing mid-IR polarimetry of silicates in astronomical sources into emissive and absorptive components. Here we extend this method to include the effects of polarization caused by scattering from graphite or similar particles along the same sightlines. To demonstrate the extended method, we apply it in the analysis of CanariCam multi-wavelength imaging polarimetry observations of the Egg Nebula, W3 IRS5, and W51 IRS2, and also spectropolarimetry of W3 IRS5. We compare these results with those obtained with the original Aitken method and show that the Egg Nebula observations are fit better when this third component is incorporated into the analysis. Polarimetry observations of W3 IRS5 are also fit better with the extended Aitken method, but the original method suffices to fit many sightlines. Observations of W51 IRS2 are fit well by either the original or extended Aitken method. Including scattering by dust in the decomposition of polarimetry observations of the Egg Nebula and W3 IRS5 produces better results for the emissive and absorptive components, and in particular for the position angle (PA) of those components. The distribution of the difference between absorptive and emissive PA is then found to be more peaked at a single angle, nearly perpendicular. This supports the theory that mid-IR polarization arises from elongated dust grains aligned along magnetic field lines, since then the PA of emissive and absorptive polarization would be perpendicular. When significant scattering is not present the extended method produces the same results as the original method.