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 A(V) similar to 48. We find that P-K/tau(K) 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 A(V) greater than or similar to 20 the slope for P versus tau becomes similar to-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(V) similar to 20. A simple model relating the alignment efficiency to the optical depth into the cloud reproduces the observations well.