2007ApJ...665..369A

We present new multicolor photopolarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, ρ Opiuchus, R CrA, and Taurus clouds, we show that the wavelength of maximum polarization (λ_max) is linearly correlated with the radiation environment of the grains. Using far-infrared emission data, we show that the large scatter seen in previous studies of λ_max as a function of A_Vis primarily due to line-of-sight effects causing some A_V measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in λ_max versus A_V, for the individual clouds, are consistent with a common value, while the zero intercepts scale with the average values of the ratios of total to selective extinction (R_V) for the individual clouds. Within each cloud we do not find direct correlations between λ_max and R_V. The positive slope is consistent with recent developments in theory and indicating alignment driven by the radiation field. The present data cannot conclusively differentiate between direct radiative torques and alignment driven by H₂formation. However, the small values of λ_max(A_V=0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the λ_max versus A_Vrelation is found to be associated with the characteristics of the embedded YSOs in the clouds. We propose that this is partially due to locally increased plasma damping of the grain rotation caused by X-rays from the YSOs.