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Astron. Astrophys. 344, 668-674 (1999)

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6. Conclusions

Polarization data at 800 and 1100 [FORMULA] have been obtained for Mon R2, DR21 and W3-IRS4. The results imply that submillimetre polarization is dominated by grain alignment, rather than heating effects; however, source radiation may be important for the spin-up and alignment of grains. The most likely mechanisms of grain alignment are mechanical and possibly radiative processes, while superparamagnetism is unlikely, as the gas-grain temperature differences required are not present in these molecular clouds.

Examination of published data has shown that the polarization percentage and position angle can be wavelength dependent in the submillimetre. The percentage difference is almost always in the sense that p(1100) exceeds p(800), an effect which can be explained by mixtures of grains in the clouds (if less spherical grains are better aligned). The polarization is then biased towards the grain population with the most polarized flux. This can also produce a wavelength bias in the position angle, so the choice of observing wavelength in the submillimetre is not as immaterial as is frequently supposed. In the future, multi-wavelength polarimetry is desirable to give an overall picture of the magnetic field directions, and the effects of radiation on grain alignment should also be further investigated. The new Submillimetre Common-User Bolometer Array (SCUBA; Holland et al. 1999) has a multi-wavelength polarimetry mode that is now being commissioned at the JCMT, which should shortly be able to address these questions.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999
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