Grain alignment in starless cores

We present near-IR polarimetry data of background stars shining through a selection of starless cores taken in the K band, probing visual extinctions up to . We find that continues to decline with increasing A_V with a power law slope of roughly -0.5. Examination of published submillimeter (submm) polarimetry of starless cores suggests that by the slope for P versus τ becomes ∼-1, indicating no grain alignment at greater optical depths. Combining these two data sets, we find good evidence that, in the absence of a central illuminating source, the dust grains in dense molecular cloud cores with no internal radiation source cease to become aligned with the local magnetic field at optical depths greater than . A simple model relating the alignment efficiency to the optical depth into the cloud reproduces the observations well.