Open AccessGrain alignment in dense interstellar environments: spectropolarimetry of the 4.67‐μm CO‐ice feature in the field star Elias 16 (Taurus dark cloud)
Author(s) -
Hough J. H.,
Aitken D. K.,
Whittet D. C. B.,
Adamson A. J.,
Chrysostomou A.
Publication year - 2008
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2008.13274.x
Subject(s) - physics , astrophysics , radiative transfer , interstellar medium , grain size , mantle (geology) , cosmic dust , grain growth , molecular cloud , polarization (electrochemistry) , astronomy , optics , stars , geophysics , geology , geomorphology , galaxy , chemistry
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.