Grain alignment in dense interstellar environments: spectropolarimetry of the 4.67-μm CO-ice feature in the field star Elias 16 (Taurus dark cloud) |
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| Authors: | J H Hough D K Aitken D C B Whittet A J Adamson A Chrysostomou |
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| Institution: | Centre for Astrophysics Research, Science and Technology Research Centre, University of Hertfordshire, AL6 0BE;Department of Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180-3590;UK Infrared Telescope, Joint Astronomy Centre, 660 North A'Ohoku Place, Hilo, HI 96720 |
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| Abstract: | We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ～20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ～5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds. |
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| Keywords: | polarization stars: individual: Elias 16 dust extinction ISM: lines and bands ISM: magnetic fields infrared: ISM |
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