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Observational Constraints on Interstellar Grain Alignment

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 (λmax) 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 λmax as a function of AV is primarily due to line-of-sight effects causing some AV measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in λmax versus AV, 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 (RV) for the individual clouds. Within each cloud we do not find direct correlations between λmax and RV. 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 H2 formation. However, the small values of λmax(AV = 0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the λmax versus AV relation 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.