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Title:
The Efficiency of Grain Alignment in Dense Interstellar Clouds: a Reassessment of Constraints from Near-Infrared Polarization
Authors:
Whittet, D. C. B.; Hough, J. H.; Lazarian, A.; Hoang, Thiem
Affiliation:
AA(Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180; .), AB(Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK.), AC(Department of Astronomy, University of Wisconsin, Madison, WI 53706.), AD(Department of Astronomy, University of Wisconsin, Madison, WI 53706.)
Publication:
The Astrophysical Journal, Volume 674, Issue 1, pp. 304-315. (ApJ Homepage)
Publication Date:
02/2008
Origin:
UCP
ApJ Keywords:
ISM: Dust, Extinction, Infrared: ISM, ISM: Magnetic Fields, Polarization
DOI:
10.1086/525040
Bibliographic Code:
2008ApJ...674..304W

Abstract

We present the results of a detailed study of interstellar polarization efficiency (as measured by the ratio pλ/τλ) toward molecular clouds, with the aim of discriminating between grain alignment mechanisms in dense regions of the interstellar medium. The data set includes both continuum measurements in the K (2.2 μm) passband and values based on ice and silicate spectral features. Background field stars are used to probe polarization efficiency in quiescent regions of dark clouds, yielding a dependence on visual extinction well-represented by a power law (pλ/τλ~[AV]-0.52), in agreement with previous work. No significant change in this behavior is observed in the transition region between the diffuse outer layers and dense inner regions of clouds, where icy mantles are formed, and we conclude that mantle formation has little or no effect on the efficiency of grain alignment. The field-star data are used as a template for comparison with results for embedded young stellar objects (YSOs). The latter generally exhibit greater polarization efficiency compared with field stars at comparable extinctions, some displaying enhancements in pλ/τλ by factors of up to ~6 with respect to the power-law fit. Of the proposed alignment mechanisms, that based on radiative torques appears best able to explain the data. The attenuated external radiation field appears adequate to account for the observed polarization in quiescent regions for extinctions up to AV~10 mag. Radiation from the embedded stars themselves may enhance alignment in the lines of sight to YSOs. Enhancements in pλ/τλ observed in the ice features toward several YSOs are of greatest significance, as they demonstrate efficient alignment in cold molecular clouds associated with star formation.
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