Submillimeter-wavelength Polarimetry of IRC+10216

Abstract

We present SCUBA-2/POL-2 850 μ m polarimetric observations of the circumstellar envelope (CSE) of the carbon-rich asymptotic giant branch (AGB) star IRC+10216. Both far-IR (FIR) and optical polarization data indicate grains aligned with their long axis in the radial direction relative to the central star. The 850 μ m polarization does not show this simple structure. The 850 μ m data are indicative, albeit not conclusive, of a magnetic dipole geometry. Assuming such a simple dipole geometry, the resulting 850 μ m polarization geometry is consistent with both Zeeman observations and small-scale structure in the CSE. While there is significant spectral-line polarization contained within the SCUBA-2 850 μ m passband for the source, it is unlikely that our broadband polarization results are dominated by line polarization. To explain the required grain alignment, grain mineralogy effects, due to either fossil silicate grains from the earlier oxygen-rich AGB phase of the star or due to the incorporation of ferromagnetic inclusions in the largest grains, may play a role. We argue that the most likely explanation is due to a new alignment mechanism wherein a charged grain, moving relative to the magnetic field, precesses around the induced electric field and therefore aligns with the magnetic field. This mechanism is particularly attractive as the optical, FIR, and submillimeter-wave polarization of the carbon dust can then be explained in a consistent way, differing simply due to the charge state of the grains.

Keywords

• Circumstellar dust
• Magnetic fields
• Carbonaceous grains
• Asymptotic giant branch stars