The efficiency of grain alignment in dense interstellar clouds: A reassessment of constraints from near-infrared polarization

We present the results of a detailed study of interstellar polarization efficiency (as measured by the ratio p(lambda)/tau(lambda)) 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 mu m) 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 (p(lambda)/tau(lambda) proportional to [A(V)](-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 p(lambda)/tau(lambda) by factors of up to similar 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 A(V) similar to 10 mag. Radiation from the embedded stars themselves may enhance alignment in the lines of sight to YSOs. Enhancements in p(lambda)/tau(lambda) 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.