Infrared spectropolarimetry of AFGL 2591: evidence for an annealed grain component

Abstract

We present spectropolarimetric observations of AFGL 2591 in the 10 and 20 μm regions. These confirm that the polarization at these wavelengths is due to dichroic absorption by aligned grains and infer that this is also the polarization mechanism in the near infrared. The aligning field is normal to the bipolar axis of molecular flow and also to the larger scale field in the local interstellar medium. As well as the polarization feature usually seen and attributed to amorphous silicates (e.g. the BN object in Orion) an additional narrow feature is observed at 11.2 μm. It is considered that this is due to a more structured silicate, similar to olivine, which could have been produced in an annealing episode in which grains reach a temperature in excess of 1300 K. If the annealment has been produced by the central source, the material must now be expanding. The 20 μm data rule out metal oxides as significant contributors to the extinction.