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A detailed study of interstellar polarization efficiency toward molecularclouds is used to attempt discrimination between grain alignment mechanisms indense regions of the ISM. Background field stars are used to probe polarizationefficiency in quiescent regions of dark clouds, yielding a dependence on visualextinction well-represented by a power law. No significant change in thisbehavior is observed in the transition region between the diffuse outer layersand dense inner regions of clouds, where icy mantles are formed, and weconclude that mantle formation has little or no effect on the efficiency ofgrain alignment. Young stellar objects generally exhibit greater polarizationefficiency compared with field stars at comparable extinctions, displayingenhancements by factors of up to 6. Of the proposed alignment mechanisms, thatbased on radiative torques appears best able to explain the data. Theattenuated external radiation field accounts for the observed polarization inquiescent regions, and radiation from the embedded stars themselves may enhancealignment in the lines of sight to YSOs. Enhancements in polarizationefficiency observed in the ice features toward several YSOs are of greatestsignificance, as they demonstrate efficient alignment in cold molecular cloudsassociated with star formation.

The Efficiency of Grain Alignment in Dense Interstellar Clouds: A Reassessment of Constraints from Near Infrared Polarization